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240 lines
12 KiB
240 lines
12 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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========================== |
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PAT (Page Attribute Table) |
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========================== |
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x86 Page Attribute Table (PAT) allows for setting the memory attribute at the |
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page level granularity. PAT is complementary to the MTRR settings which allows |
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for setting of memory types over physical address ranges. However, PAT is |
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more flexible than MTRR due to its capability to set attributes at page level |
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and also due to the fact that there are no hardware limitations on number of |
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such attribute settings allowed. Added flexibility comes with guidelines for |
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not having memory type aliasing for the same physical memory with multiple |
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virtual addresses. |
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PAT allows for different types of memory attributes. The most commonly used |
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ones that will be supported at this time are: |
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=== ============== |
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WB Write-back |
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UC Uncached |
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WC Write-combined |
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WT Write-through |
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UC- Uncached Minus |
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=== ============== |
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PAT APIs |
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======== |
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There are many different APIs in the kernel that allows setting of memory |
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attributes at the page level. In order to avoid aliasing, these interfaces |
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should be used thoughtfully. Below is a table of interfaces available, |
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their intended usage and their memory attribute relationships. Internally, |
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these APIs use a reserve_memtype()/free_memtype() interface on the physical |
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address range to avoid any aliasing. |
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+------------------------+----------+--------------+------------------+ |
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| API | RAM | ACPI,... | Reserved/Holes | |
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+------------------------+----------+--------------+------------------+ |
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| ioremap | -- | UC- | UC- | |
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+------------------------+----------+--------------+------------------+ |
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| ioremap_cache | -- | WB | WB | |
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+------------------------+----------+--------------+------------------+ |
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| ioremap_uc | -- | UC | UC | |
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+------------------------+----------+--------------+------------------+ |
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| ioremap_wc | -- | -- | WC | |
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+------------------------+----------+--------------+------------------+ |
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| ioremap_wt | -- | -- | WT | |
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+------------------------+----------+--------------+------------------+ |
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| set_memory_uc, | UC- | -- | -- | |
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| set_memory_wb | | | | |
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+------------------------+----------+--------------+------------------+ |
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| set_memory_wc, | WC | -- | -- | |
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| set_memory_wb | | | | |
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+------------------------+----------+--------------+------------------+ |
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| set_memory_wt, | WT | -- | -- | |
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| set_memory_wb | | | | |
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+------------------------+----------+--------------+------------------+ |
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| pci sysfs resource | -- | -- | UC- | |
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+------------------------+----------+--------------+------------------+ |
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| pci sysfs resource_wc | -- | -- | WC | |
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| is IORESOURCE_PREFETCH | | | | |
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+------------------------+----------+--------------+------------------+ |
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| pci proc | -- | -- | UC- | |
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| !PCIIOC_WRITE_COMBINE | | | | |
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+------------------------+----------+--------------+------------------+ |
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| pci proc | -- | -- | WC | |
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| PCIIOC_WRITE_COMBINE | | | | |
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+------------------------+----------+--------------+------------------+ |
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| /dev/mem | -- | WB/WC/UC- | WB/WC/UC- | |
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| read-write | | | | |
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+------------------------+----------+--------------+------------------+ |
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| /dev/mem | -- | UC- | UC- | |
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| mmap SYNC flag | | | | |
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+------------------------+----------+--------------+------------------+ |
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| /dev/mem | -- | WB/WC/UC- | WB/WC/UC- | |
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| mmap !SYNC flag | | | | |
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| and | |(from existing| (from existing | |
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| any alias to this area | |alias) | alias) | |
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+------------------------+----------+--------------+------------------+ |
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| /dev/mem | -- | WB | WB | |
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| mmap !SYNC flag | | | | |
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| no alias to this area | | | | |
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| and | | | | |
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| MTRR says WB | | | | |
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+------------------------+----------+--------------+------------------+ |
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| /dev/mem | -- | -- | UC- | |
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| mmap !SYNC flag | | | | |
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| no alias to this area | | | | |
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| and | | | | |
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| MTRR says !WB | | | | |
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+------------------------+----------+--------------+------------------+ |
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Advanced APIs for drivers |
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========================= |
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A. Exporting pages to users with remap_pfn_range, io_remap_pfn_range, |
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vmf_insert_pfn. |
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Drivers wanting to export some pages to userspace do it by using mmap |
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interface and a combination of: |
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1) pgprot_noncached() |
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2) io_remap_pfn_range() or remap_pfn_range() or vmf_insert_pfn() |
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With PAT support, a new API pgprot_writecombine is being added. So, drivers can |
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continue to use the above sequence, with either pgprot_noncached() or |
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pgprot_writecombine() in step 1, followed by step 2. |
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In addition, step 2 internally tracks the region as UC or WC in memtype |
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list in order to ensure no conflicting mapping. |
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Note that this set of APIs only works with IO (non RAM) regions. If driver |
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wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc() |
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as step 0 above and also track the usage of those pages and use set_memory_wb() |
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before the page is freed to free pool. |
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MTRR effects on PAT / non-PAT systems |
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===================================== |
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The following table provides the effects of using write-combining MTRRs when |
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using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally |
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mtrr_add() usage will be phased out in favor of arch_phys_wc_add() which will |
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be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add() |
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is made, should already have been ioremapped with WC attributes or PAT entries, |
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this can be done by using ioremap_wc() / set_memory_wc(). Devices which |
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combine areas of IO memory desired to remain uncacheable with areas where |
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write-combining is desirable should consider use of ioremap_uc() followed by |
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set_memory_wc() to white-list effective write-combined areas. Such use is |
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nevertheless discouraged as the effective memory type is considered |
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implementation defined, yet this strategy can be used as last resort on devices |
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with size-constrained regions where otherwise MTRR write-combining would |
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otherwise not be effective. |
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:: |
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==== ======= === ========================= ===================== |
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MTRR Non-PAT PAT Linux ioremap value Effective memory type |
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==== ======= === ========================= ===================== |
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PAT Non-PAT | PAT |
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|PCD | |
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||PWT | |
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WC 000 WB _PAGE_CACHE_MODE_WB WC | WC |
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WC 001 WC _PAGE_CACHE_MODE_WC WC* | WC |
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WC 010 UC- _PAGE_CACHE_MODE_UC_MINUS WC* | UC |
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WC 011 UC _PAGE_CACHE_MODE_UC UC | UC |
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==== ======= === ========================= ===================== |
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(*) denotes implementation defined and is discouraged |
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.. note:: -- in the above table mean "Not suggested usage for the API". Some |
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of the --'s are strictly enforced by the kernel. Some others are not really |
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enforced today, but may be enforced in future. |
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For ioremap and pci access through /sys or /proc - The actual type returned |
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can be more restrictive, in case of any existing aliasing for that address. |
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For example: If there is an existing uncached mapping, a new ioremap_wc can |
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return uncached mapping in place of write-combine requested. |
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set_memory_[uc|wc|wt] and set_memory_wb should be used in pairs, where driver |
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will first make a region uc, wc or wt and switch it back to wb after use. |
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Over time writes to /proc/mtrr will be deprecated in favor of using PAT based |
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interfaces. Users writing to /proc/mtrr are suggested to use above interfaces. |
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Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access |
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types. |
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Drivers should use set_memory_[uc|wc|wt] to set access type for RAM ranges. |
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PAT debugging |
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============= |
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With CONFIG_DEBUG_FS enabled, PAT memtype list can be examined by:: |
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# mount -t debugfs debugfs /sys/kernel/debug |
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# cat /sys/kernel/debug/x86/pat_memtype_list |
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PAT memtype list: |
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uncached-minus @ 0x7fadf000-0x7fae0000 |
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uncached-minus @ 0x7fb19000-0x7fb1a000 |
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uncached-minus @ 0x7fb1a000-0x7fb1b000 |
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uncached-minus @ 0x7fb1b000-0x7fb1c000 |
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uncached-minus @ 0x7fb1c000-0x7fb1d000 |
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uncached-minus @ 0x7fb1d000-0x7fb1e000 |
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uncached-minus @ 0x7fb1e000-0x7fb25000 |
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uncached-minus @ 0x7fb25000-0x7fb26000 |
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uncached-minus @ 0x7fb26000-0x7fb27000 |
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uncached-minus @ 0x7fb27000-0x7fb28000 |
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uncached-minus @ 0x7fb28000-0x7fb2e000 |
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uncached-minus @ 0x7fb2e000-0x7fb2f000 |
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uncached-minus @ 0x7fb2f000-0x7fb30000 |
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uncached-minus @ 0x7fb31000-0x7fb32000 |
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uncached-minus @ 0x80000000-0x90000000 |
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This list shows physical address ranges and various PAT settings used to |
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access those physical address ranges. |
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Another, more verbose way of getting PAT related debug messages is with |
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"debugpat" boot parameter. With this parameter, various debug messages are |
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printed to dmesg log. |
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PAT Initialization |
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================== |
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The following table describes how PAT is initialized under various |
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configurations. The PAT MSR must be updated by Linux in order to support WC |
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and WT attributes. Otherwise, the PAT MSR has the value programmed in it |
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by the firmware. Note, Xen enables WC attribute in the PAT MSR for guests. |
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==== ===== ========================== ========= ======= |
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MTRR PAT Call Sequence PAT State PAT MSR |
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==== ===== ========================== ========= ======= |
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E E MTRR -> PAT init Enabled OS |
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E D MTRR -> PAT init Disabled - |
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D E MTRR -> PAT disable Disabled BIOS |
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D D MTRR -> PAT disable Disabled - |
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- np/E PAT -> PAT disable Disabled BIOS |
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- np/D PAT -> PAT disable Disabled - |
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E !P/E MTRR -> PAT init Disabled BIOS |
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D !P/E MTRR -> PAT disable Disabled BIOS |
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!M !P/E MTRR stub -> PAT disable Disabled BIOS |
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==== ===== ========================== ========= ======= |
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Legend |
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========= ======================================= |
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E Feature enabled in CPU |
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D Feature disabled/unsupported in CPU |
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np "nopat" boot option specified |
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!P CONFIG_X86_PAT option unset |
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!M CONFIG_MTRR option unset |
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Enabled PAT state set to enabled |
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Disabled PAT state set to disabled |
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OS PAT initializes PAT MSR with OS setting |
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BIOS PAT keeps PAT MSR with BIOS setting |
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========= ======================================= |
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