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249 lines
12 KiB
249 lines
12 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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================= |
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KVM Lock Overview |
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================= |
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1. Acquisition Orders |
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--------------------- |
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The acquisition orders for mutexes are as follows: |
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- kvm->lock is taken outside vcpu->mutex |
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- kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock |
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- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring |
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them together is quite rare. |
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On x86: |
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- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock |
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- kvm->arch.mmu_lock is an rwlock. kvm->arch.tdp_mmu_pages_lock is |
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taken inside kvm->arch.mmu_lock, and cannot be taken without already |
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holding kvm->arch.mmu_lock (typically with ``read_lock``, otherwise |
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there's no need to take kvm->arch.tdp_mmu_pages_lock at all). |
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Everything else is a leaf: no other lock is taken inside the critical |
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sections. |
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2. Exception |
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------------ |
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Fast page fault: |
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Fast page fault is the fast path which fixes the guest page fault out of |
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the mmu-lock on x86. Currently, the page fault can be fast in one of the |
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following two cases: |
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1. Access Tracking: The SPTE is not present, but it is marked for access |
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tracking i.e. the SPTE_SPECIAL_MASK is set. That means we need to |
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restore the saved R/X bits. This is described in more detail later below. |
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2. Write-Protection: The SPTE is present and the fault is |
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caused by write-protect. That means we just need to change the W bit of |
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the spte. |
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What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and |
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SPTE_MMU_WRITEABLE bit on the spte: |
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- SPTE_HOST_WRITEABLE means the gfn is writable on host. |
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- SPTE_MMU_WRITEABLE means the gfn is writable on mmu. The bit is set when |
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the gfn is writable on guest mmu and it is not write-protected by shadow |
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page write-protection. |
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On fast page fault path, we will use cmpxchg to atomically set the spte W |
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bit if spte.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, or |
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restore the saved R/X bits if VMX_EPT_TRACK_ACCESS mask is set, or both. This |
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is safe because whenever changing these bits can be detected by cmpxchg. |
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But we need carefully check these cases: |
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1) The mapping from gfn to pfn |
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The mapping from gfn to pfn may be changed since we can only ensure the pfn |
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is not changed during cmpxchg. This is a ABA problem, for example, below case |
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will happen: |
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+------------------------------------------------------------------------+ |
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| At the beginning:: | |
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| | |
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| gpte = gfn1 | |
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| gfn1 is mapped to pfn1 on host | |
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| spte is the shadow page table entry corresponding with gpte and | |
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| spte = pfn1 | |
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+------------------------------------------------------------------------+ |
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| On fast page fault path: | |
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+------------------------------------+-----------------------------------+ |
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| CPU 0: | CPU 1: | |
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+------------------------------------+-----------------------------------+ |
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| :: | | |
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| | | |
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| old_spte = *spte; | | |
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+------------------------------------+-----------------------------------+ |
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| | pfn1 is swapped out:: | |
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| | | |
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| | spte = 0; | |
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| | | |
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| | pfn1 is re-alloced for gfn2. | |
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| | | |
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| | gpte is changed to point to | |
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| | gfn2 by the guest:: | |
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| | | |
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| | spte = pfn1; | |
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+------------------------------------+-----------------------------------+ |
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| :: | |
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| if (cmpxchg(spte, old_spte, old_spte+W) | |
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| mark_page_dirty(vcpu->kvm, gfn1) | |
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| OOPS!!! | |
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+------------------------------------------------------------------------+ |
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We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap. |
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For direct sp, we can easily avoid it since the spte of direct sp is fixed |
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to gfn. For indirect sp, we disabled fast page fault for simplicity. |
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A solution for indirect sp could be to pin the gfn, for example via |
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kvm_vcpu_gfn_to_pfn_atomic, before the cmpxchg. After the pinning: |
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- We have held the refcount of pfn that means the pfn can not be freed and |
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be reused for another gfn. |
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- The pfn is writable and therefore it cannot be shared between different gfns |
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by KSM. |
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Then, we can ensure the dirty bitmaps is correctly set for a gfn. |
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2) Dirty bit tracking |
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In the origin code, the spte can be fast updated (non-atomically) if the |
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spte is read-only and the Accessed bit has already been set since the |
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Accessed bit and Dirty bit can not be lost. |
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But it is not true after fast page fault since the spte can be marked |
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writable between reading spte and updating spte. Like below case: |
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+------------------------------------------------------------------------+ |
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| At the beginning:: | |
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| | |
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| spte.W = 0 | |
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| spte.Accessed = 1 | |
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+------------------------------------+-----------------------------------+ |
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| CPU 0: | CPU 1: | |
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+------------------------------------+-----------------------------------+ |
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| In mmu_spte_clear_track_bits():: | | |
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| old_spte = *spte; | | |
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| /* 'if' condition is satisfied. */| | |
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| if (old_spte.Accessed == 1 && | | |
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| old_spte.W == 0) | | |
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| spte = 0ull; | | |
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+------------------------------------+-----------------------------------+ |
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| | on fast page fault path:: | |
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| | | |
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| | spte.W = 1 | |
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| | memory write on the spte:: | |
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| | spte.Dirty = 1 | |
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+------------------------------------+-----------------------------------+ |
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| :: | | |
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| else | | |
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| old_spte = xchg(spte, 0ull) | | |
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| if (old_spte.Accessed == 1) | | |
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| kvm_set_pfn_accessed(spte.pfn);| | |
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| if (old_spte.Dirty == 1) | | |
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| kvm_set_pfn_dirty(spte.pfn); | | |
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| OOPS!!! | | |
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+------------------------------------+-----------------------------------+ |
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The Dirty bit is lost in this case. |
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In order to avoid this kind of issue, we always treat the spte as "volatile" |
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if it can be updated out of mmu-lock, see spte_has_volatile_bits(), it means, |
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the spte is always atomically updated in this case. |
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3) flush tlbs due to spte updated |
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If the spte is updated from writable to readonly, we should flush all TLBs, |
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otherwise rmap_write_protect will find a read-only spte, even though the |
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writable spte might be cached on a CPU's TLB. |
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As mentioned before, the spte can be updated to writable out of mmu-lock on |
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fast page fault path, in order to easily audit the path, we see if TLBs need |
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be flushed caused by this reason in mmu_spte_update() since this is a common |
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function to update spte (present -> present). |
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Since the spte is "volatile" if it can be updated out of mmu-lock, we always |
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atomically update the spte, the race caused by fast page fault can be avoided, |
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See the comments in spte_has_volatile_bits() and mmu_spte_update(). |
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Lockless Access Tracking: |
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This is used for Intel CPUs that are using EPT but do not support the EPT A/D |
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bits. In this case, when the KVM MMU notifier is called to track accesses to a |
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page (via kvm_mmu_notifier_clear_flush_young), it marks the PTE as not-present |
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by clearing the RWX bits in the PTE and storing the original R & X bits in |
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some unused/ignored bits. In addition, the SPTE_SPECIAL_MASK is also set on the |
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PTE (using the ignored bit 62). When the VM tries to access the page later on, |
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a fault is generated and the fast page fault mechanism described above is used |
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to atomically restore the PTE to a Present state. The W bit is not saved when |
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the PTE is marked for access tracking and during restoration to the Present |
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state, the W bit is set depending on whether or not it was a write access. If |
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it wasn't, then the W bit will remain clear until a write access happens, at |
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which time it will be set using the Dirty tracking mechanism described above. |
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3. Reference |
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------------ |
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:Name: kvm_lock |
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:Type: mutex |
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:Arch: any |
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:Protects: - vm_list |
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:Name: kvm_count_lock |
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:Type: raw_spinlock_t |
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:Arch: any |
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:Protects: - hardware virtualization enable/disable |
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:Comment: 'raw' because hardware enabling/disabling must be atomic /wrt |
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migration. |
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:Name: kvm_arch::tsc_write_lock |
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:Type: raw_spinlock |
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:Arch: x86 |
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:Protects: - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset} |
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- tsc offset in vmcb |
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:Comment: 'raw' because updating the tsc offsets must not be preempted. |
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:Name: kvm->mmu_lock |
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:Type: spinlock_t |
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:Arch: any |
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:Protects: -shadow page/shadow tlb entry |
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:Comment: it is a spinlock since it is used in mmu notifier. |
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:Name: kvm->srcu |
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:Type: srcu lock |
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:Arch: any |
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:Protects: - kvm->memslots |
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- kvm->buses |
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:Comment: The srcu read lock must be held while accessing memslots (e.g. |
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when using gfn_to_* functions) and while accessing in-kernel |
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MMIO/PIO address->device structure mapping (kvm->buses). |
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The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu |
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if it is needed by multiple functions. |
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:Name: blocked_vcpu_on_cpu_lock |
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:Type: spinlock_t |
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:Arch: x86 |
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:Protects: blocked_vcpu_on_cpu |
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:Comment: This is a per-CPU lock and it is used for VT-d posted-interrupts. |
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When VT-d posted-interrupts is supported and the VM has assigned |
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devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu |
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protected by blocked_vcpu_on_cpu_lock, when VT-d hardware issues |
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wakeup notification event since external interrupts from the |
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assigned devices happens, we will find the vCPU on the list to |
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wakeup.
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