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428 lines
16 KiB
428 lines
16 KiB
=================================================== |
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Scalable Matrix Extension support for AArch64 Linux |
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=================================================== |
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This document outlines briefly the interface provided to userspace by Linux in |
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order to support use of the ARM Scalable Matrix Extension (SME). |
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This is an outline of the most important features and issues only and not |
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intended to be exhaustive. It should be read in conjunction with the SVE |
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documentation in sve.rst which provides details on the Streaming SVE mode |
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included in SME. |
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This document does not aim to describe the SME architecture or programmer's |
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model. To aid understanding, a minimal description of relevant programmer's |
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model features for SME is included in Appendix A. |
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1. General |
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----------- |
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* PSTATE.SM, PSTATE.ZA, the streaming mode vector length, the ZA |
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register state and TPIDR2_EL0 are tracked per thread. |
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* The presence of SME is reported to userspace via HWCAP2_SME in the aux vector |
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AT_HWCAP2 entry. Presence of this flag implies the presence of the SME |
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instructions and registers, and the Linux-specific system interfaces |
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described in this document. SME is reported in /proc/cpuinfo as "sme". |
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* Support for the execution of SME instructions in userspace can also be |
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detected by reading the CPU ID register ID_AA64PFR1_EL1 using an MRS |
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instruction, and checking that the value of the SME field is nonzero. [3] |
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It does not guarantee the presence of the system interfaces described in the |
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following sections: software that needs to verify that those interfaces are |
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present must check for HWCAP2_SME instead. |
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* There are a number of optional SME features, presence of these is reported |
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through AT_HWCAP2 through: |
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HWCAP2_SME_I16I64 |
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HWCAP2_SME_F64F64 |
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HWCAP2_SME_I8I32 |
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HWCAP2_SME_F16F32 |
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HWCAP2_SME_B16F32 |
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HWCAP2_SME_F32F32 |
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HWCAP2_SME_FA64 |
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This list may be extended over time as the SME architecture evolves. |
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These extensions are also reported via the CPU ID register ID_AA64SMFR0_EL1, |
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which userspace can read using an MRS instruction. See elf_hwcaps.txt and |
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cpu-feature-registers.txt for details. |
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* Debuggers should restrict themselves to interacting with the target via the |
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NT_ARM_SVE, NT_ARM_SSVE and NT_ARM_ZA regsets. The recommended way |
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of detecting support for these regsets is to connect to a target process |
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first and then attempt a |
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ptrace(PTRACE_GETREGSET, pid, NT_ARM_<regset>, &iov). |
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* Whenever ZA register values are exchanged in memory between userspace and |
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the kernel, the register value is encoded in memory as a series of horizontal |
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vectors from 0 to VL/8-1 stored in the same endianness invariant format as is |
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used for SVE vectors. |
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* On thread creation TPIDR2_EL0 is preserved unless CLONE_SETTLS is specified, |
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in which case it is set to 0. |
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2. Vector lengths |
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------------------ |
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SME defines a second vector length similar to the SVE vector length which is |
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controls the size of the streaming mode SVE vectors and the ZA matrix array. |
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The ZA matrix is square with each side having as many bytes as a streaming |
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mode SVE vector. |
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3. Sharing of streaming and non-streaming mode SVE state |
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--------------------------------------------------------- |
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It is implementation defined which if any parts of the SVE state are shared |
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between streaming and non-streaming modes. When switching between modes |
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via software interfaces such as ptrace if no register content is provided as |
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part of switching no state will be assumed to be shared and everything will |
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be zeroed. |
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4. System call behaviour |
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------------------------- |
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* On syscall PSTATE.ZA is preserved, if PSTATE.ZA==1 then the contents of the |
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ZA matrix are preserved. |
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* On syscall PSTATE.SM will be cleared and the SVE registers will be handled |
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as per the standard SVE ABI. |
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* Neither the SVE registers nor ZA are used to pass arguments to or receive |
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results from any syscall. |
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* On process creation (eg, clone()) the newly created process will have |
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PSTATE.SM cleared. |
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* All other SME state of a thread, including the currently configured vector |
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length, the state of the PR_SME_VL_INHERIT flag, and the deferred vector |
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length (if any), is preserved across all syscalls, subject to the specific |
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exceptions for execve() described in section 6. |
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5. Signal handling |
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------------------- |
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* Signal handlers are invoked with streaming mode and ZA disabled. |
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* A new signal frame record za_context encodes the ZA register contents on |
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signal delivery. [1] |
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* The signal frame record for ZA always contains basic metadata, in particular |
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the thread's vector length (in za_context.vl). |
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* The ZA matrix may or may not be included in the record, depending on |
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the value of PSTATE.ZA. The registers are present if and only if: |
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za_context.head.size >= ZA_SIG_CONTEXT_SIZE(sve_vq_from_vl(za_context.vl)) |
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in which case PSTATE.ZA == 1. |
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* If matrix data is present, the remainder of the record has a vl-dependent |
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size and layout. Macros ZA_SIG_* are defined [1] to facilitate access to |
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them. |
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* The matrix is stored as a series of horizontal vectors in the same format as |
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is used for SVE vectors. |
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* If the ZA context is too big to fit in sigcontext.__reserved[], then extra |
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space is allocated on the stack, an extra_context record is written in |
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__reserved[] referencing this space. za_context is then written in the |
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extra space. Refer to [1] for further details about this mechanism. |
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5. Signal return |
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----------------- |
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When returning from a signal handler: |
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* If there is no za_context record in the signal frame, or if the record is |
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present but contains no register data as described in the previous section, |
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then ZA is disabled. |
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* If za_context is present in the signal frame and contains matrix data then |
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PSTATE.ZA is set to 1 and ZA is populated with the specified data. |
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* The vector length cannot be changed via signal return. If za_context.vl in |
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the signal frame does not match the current vector length, the signal return |
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attempt is treated as illegal, resulting in a forced SIGSEGV. |
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6. prctl extensions |
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-------------------- |
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Some new prctl() calls are added to allow programs to manage the SME vector |
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length: |
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prctl(PR_SME_SET_VL, unsigned long arg) |
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Sets the vector length of the calling thread and related flags, where |
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arg == vl | flags. Other threads of the calling process are unaffected. |
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vl is the desired vector length, where sve_vl_valid(vl) must be true. |
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flags: |
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PR_SME_VL_INHERIT |
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Inherit the current vector length across execve(). Otherwise, the |
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vector length is reset to the system default at execve(). (See |
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Section 9.) |
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PR_SME_SET_VL_ONEXEC |
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Defer the requested vector length change until the next execve() |
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performed by this thread. |
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The effect is equivalent to implicit execution of the following |
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call immediately after the next execve() (if any) by the thread: |
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prctl(PR_SME_SET_VL, arg & ~PR_SME_SET_VL_ONEXEC) |
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This allows launching of a new program with a different vector |
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length, while avoiding runtime side effects in the caller. |
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Without PR_SME_SET_VL_ONEXEC, the requested change takes effect |
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immediately. |
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Return value: a nonnegative on success, or a negative value on error: |
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EINVAL: SME not supported, invalid vector length requested, or |
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invalid flags. |
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On success: |
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* Either the calling thread's vector length or the deferred vector length |
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to be applied at the next execve() by the thread (dependent on whether |
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PR_SME_SET_VL_ONEXEC is present in arg), is set to the largest value |
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supported by the system that is less than or equal to vl. If vl == |
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SVE_VL_MAX, the value set will be the largest value supported by the |
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system. |
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* Any previously outstanding deferred vector length change in the calling |
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thread is cancelled. |
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* The returned value describes the resulting configuration, encoded as for |
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PR_SME_GET_VL. The vector length reported in this value is the new |
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current vector length for this thread if PR_SME_SET_VL_ONEXEC was not |
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present in arg; otherwise, the reported vector length is the deferred |
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vector length that will be applied at the next execve() by the calling |
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thread. |
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* Changing the vector length causes all of ZA, P0..P15, FFR and all bits of |
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Z0..Z31 except for Z0 bits [127:0] .. Z31 bits [127:0] to become |
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unspecified, including both streaming and non-streaming SVE state. |
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Calling PR_SME_SET_VL with vl equal to the thread's current vector |
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length, or calling PR_SME_SET_VL with the PR_SVE_SET_VL_ONEXEC flag, |
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does not constitute a change to the vector length for this purpose. |
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* Changing the vector length causes PSTATE.ZA and PSTATE.SM to be cleared. |
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Calling PR_SME_SET_VL with vl equal to the thread's current vector |
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length, or calling PR_SME_SET_VL with the PR_SVE_SET_VL_ONEXEC flag, |
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does not constitute a change to the vector length for this purpose. |
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prctl(PR_SME_GET_VL) |
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Gets the vector length of the calling thread. |
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The following flag may be OR-ed into the result: |
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PR_SME_VL_INHERIT |
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Vector length will be inherited across execve(). |
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There is no way to determine whether there is an outstanding deferred |
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vector length change (which would only normally be the case between a |
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fork() or vfork() and the corresponding execve() in typical use). |
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To extract the vector length from the result, bitwise and it with |
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PR_SME_VL_LEN_MASK. |
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Return value: a nonnegative value on success, or a negative value on error: |
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EINVAL: SME not supported. |
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7. ptrace extensions |
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--------------------- |
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* A new regset NT_ARM_SSVE is defined for access to streaming mode SVE |
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state via PTRACE_GETREGSET and PTRACE_SETREGSET, this is documented in |
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sve.rst. |
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* A new regset NT_ARM_ZA is defined for ZA state for access to ZA state via |
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PTRACE_GETREGSET and PTRACE_SETREGSET. |
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Refer to [2] for definitions. |
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The regset data starts with struct user_za_header, containing: |
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size |
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Size of the complete regset, in bytes. |
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This depends on vl and possibly on other things in the future. |
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If a call to PTRACE_GETREGSET requests less data than the value of |
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size, the caller can allocate a larger buffer and retry in order to |
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read the complete regset. |
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max_size |
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Maximum size in bytes that the regset can grow to for the target |
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thread. The regset won't grow bigger than this even if the target |
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thread changes its vector length etc. |
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vl |
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Target thread's current streaming vector length, in bytes. |
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max_vl |
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Maximum possible streaming vector length for the target thread. |
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flags |
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Zero or more of the following flags, which have the same |
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meaning and behaviour as the corresponding PR_SET_VL_* flags: |
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SME_PT_VL_INHERIT |
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SME_PT_VL_ONEXEC (SETREGSET only). |
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* The effects of changing the vector length and/or flags are equivalent to |
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those documented for PR_SME_SET_VL. |
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The caller must make a further GETREGSET call if it needs to know what VL is |
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actually set by SETREGSET, unless is it known in advance that the requested |
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VL is supported. |
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* The size and layout of the payload depends on the header fields. The |
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SME_PT_ZA_*() macros are provided to facilitate access to the data. |
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* In either case, for SETREGSET it is permissible to omit the payload, in which |
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case the vector length and flags are changed and PSTATE.ZA is set to 0 |
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(along with any consequences of those changes). If a payload is provided |
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then PSTATE.ZA will be set to 1. |
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* For SETREGSET, if the requested VL is not supported, the effect will be the |
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same as if the payload were omitted, except that an EIO error is reported. |
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No attempt is made to translate the payload data to the correct layout |
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for the vector length actually set. It is up to the caller to translate the |
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payload layout for the actual VL and retry. |
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* The effect of writing a partial, incomplete payload is unspecified. |
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8. ELF coredump extensions |
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--------------------------- |
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* NT_ARM_SSVE notes will be added to each coredump for |
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each thread of the dumped process. The contents will be equivalent to the |
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data that would have been read if a PTRACE_GETREGSET of the corresponding |
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type were executed for each thread when the coredump was generated. |
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* A NT_ARM_ZA note will be added to each coredump for each thread of the |
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dumped process. The contents will be equivalent to the data that would have |
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been read if a PTRACE_GETREGSET of NT_ARM_ZA were executed for each thread |
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when the coredump was generated. |
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9. System runtime configuration |
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-------------------------------- |
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* To mitigate the ABI impact of expansion of the signal frame, a policy |
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mechanism is provided for administrators, distro maintainers and developers |
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to set the default vector length for userspace processes: |
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/proc/sys/abi/sme_default_vector_length |
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Writing the text representation of an integer to this file sets the system |
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default vector length to the specified value, unless the value is greater |
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than the maximum vector length supported by the system in which case the |
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default vector length is set to that maximum. |
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The result can be determined by reopening the file and reading its |
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contents. |
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At boot, the default vector length is initially set to 32 or the maximum |
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supported vector length, whichever is smaller and supported. This |
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determines the initial vector length of the init process (PID 1). |
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Reading this file returns the current system default vector length. |
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* At every execve() call, the new vector length of the new process is set to |
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the system default vector length, unless |
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* PR_SME_VL_INHERIT (or equivalently SME_PT_VL_INHERIT) is set for the |
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calling thread, or |
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* a deferred vector length change is pending, established via the |
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PR_SME_SET_VL_ONEXEC flag (or SME_PT_VL_ONEXEC). |
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* Modifying the system default vector length does not affect the vector length |
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of any existing process or thread that does not make an execve() call. |
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Appendix A. SME programmer's model (informative) |
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================================================= |
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This section provides a minimal description of the additions made by SME to the |
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ARMv8-A programmer's model that are relevant to this document. |
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Note: This section is for information only and not intended to be complete or |
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to replace any architectural specification. |
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A.1. Registers |
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--------------- |
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In A64 state, SME adds the following: |
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* A new mode, streaming mode, in which a subset of the normal FPSIMD and SVE |
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features are available. When supported EL0 software may enter and leave |
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streaming mode at any time. |
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For best system performance it is strongly encouraged for software to enable |
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streaming mode only when it is actively being used. |
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* A new vector length controlling the size of ZA and the Z registers when in |
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streaming mode, separately to the vector length used for SVE when not in |
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streaming mode. There is no requirement that either the currently selected |
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vector length or the set of vector lengths supported for the two modes in |
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a given system have any relationship. The streaming mode vector length |
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is referred to as SVL. |
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* A new ZA matrix register. This is a square matrix of SVLxSVL bits. Most |
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operations on ZA require that streaming mode be enabled but ZA can be |
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enabled without streaming mode in order to load, save and retain data. |
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For best system performance it is strongly encouraged for software to enable |
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ZA only when it is actively being used. |
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* Two new 1 bit fields in PSTATE which may be controlled via the SMSTART and |
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SMSTOP instructions or by access to the SVCR system register: |
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* PSTATE.ZA, if this is 1 then the ZA matrix is accessible and has valid |
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data while if it is 0 then ZA can not be accessed. When PSTATE.ZA is |
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changed from 0 to 1 all bits in ZA are cleared. |
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* PSTATE.SM, if this is 1 then the PE is in streaming mode. When the value |
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of PSTATE.SM is changed then it is implementation defined if the subset |
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of the floating point register bits valid in both modes may be retained. |
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Any other bits will be cleared. |
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References |
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========== |
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[1] arch/arm64/include/uapi/asm/sigcontext.h |
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AArch64 Linux signal ABI definitions |
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[2] arch/arm64/include/uapi/asm/ptrace.h |
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AArch64 Linux ptrace ABI definitions |
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[3] Documentation/arm64/cpu-feature-registers.rst
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