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