Message ID | 20230609130140.182781-2-janusz.krzysztofik@linux.intel.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | [v3] x86/mm: Fix PAT bit missing from page protection modify mask | expand |
On 09.06.23 15:01, Janusz Krzysztofik wrote: > Visible glitches have been observed when running graphics applications on > Linux under Xen hypervisor. Those observations have been confirmed with > failures from kms_pwrite_crc Intel GPU test that verifies data coherency > of DRM frame buffer objects using hardware CRC checksums calculated by > display controllers, exposed to userspace via debugfs. Affected > processing paths have then been identified with new IGT test variants that > mmap the objects using different methods and caching modes [1]. > > When running as a Xen PV guest, Linux uses Xen provided PAT configuration > which is different from its native one. In particular, Xen specific PTE > encoding of write-combining caching, likely used by graphics applications, > differs from the Linux default one found among statically defined minimal > set of supported modes. Since Xen defines PTE encoding of the WC mode as > _PAGE_PAT, it no longer belongs to the minimal set, depends on correct > handling of _PAGE_PAT bit, and can be mismatched with write-back caching. > > When a user calls mmap() for a DRM buffer object, DRM device specific > .mmap file operation, called from mmap_region(), takes care of setting PTE > encoding bits in a vm_page_prot field of an associated virtual memory area > structure. Unfortunately, _PAGE_PAT bit is not preserved when the vma's > .vm_flags are then applied to .vm_page_prot via vm_set_page_prot(). Bits > to be preserved are determined with _PAGE_CHG_MASK symbol that doesn't > cover _PAGE_PAT. As a consequence, WB caching is requested instead of WC > when running under Xen (also, WP is silently changed to WT, and UC > downgraded to UC_MINUS). When running on bare metal, WC is not affected, > but WP and WT extra modes are unintentionally replaced with WC and UC, > respectively. > > WP and WT modes, encoded with _PAGE_PAT bit set, were introduced by commit > 281d4078bec3 ("x86: Make page cache mode a real type"). Care was taken > to extend _PAGE_CACHE_MASK symbol with that additional bit, but that > symbol has never been used for identification of bits preserved when > applying page protection flags. Support for all cache modes under Xen, > including the problematic WC mode, was then introduced by commit > 47591df50512 ("xen: Support Xen pv-domains using PAT"). > > The issue needs to be fixed by including _PAGE_PAT bit into a bitmask used > by pgprot_modify() for selecting bits to be preserved. We can do that > either internally to pgprot_modify() (as initially proposed), or by making > _PAGE_PAT a part of _PAGE_CHG_MASK. If we go for the latter then, since > _PAGE_PAT is the same as _PAGE_PSE, we need to note that _HPAGE_CHG_MASK > -- a huge pmds' counterpart of _PAGE_CHG_MASK, introduced by commit > c489f1257b8c ("thp: add pmd_modify"), defined as (_PAGE_CHG_MASK | > _PAGE_PSE) -- will no longer differ from _PAGE_CHG_MASK. If such > modification of _PAGE_CHG_MASK was irrelevant to its users then one might > wonder why that new _HPAGE_CHG_MASK symbol was introduced instead of > reusing the existing one with that otherwise irrelevant bit (_PAGE_PSE in > that case) added. > > Assume that adding _PAGE_PAT to _PAGE_CHG_MASK doesn't break pte_modify() > and its users, and go for it. Also, add _PAGE_PAT_LARGE to > _HPAGE_CHG_MASK for symmetry. For better clarity, split out common bits > from both symbols to another one and use it together with specific bits > when defining the masks. > > v3: Separate out common bits of _PAGE_CHG_MASK and _HPAGE_CHG_MASK into > _COMMON_PAGE_CHG_MASK (Rick), > - fix hard to parse wording of 'what' part of commit description (on > Dave's request). > v2: Keep pgprot_modify() untouched, make _PAGE_PAT part of _PAGE_CHG_MASK > instead (Borislav), > - also add _PAGE_PAT_LARGE to _HPAGE_CHG_MASK (Juergen). > > [1] https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/commit/0f0754413f14 > > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7648 > Fixes: 281d4078bec3 ("x86: Make page cache mode a real type") > Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com> > Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com> > Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com> > Acked-by: Juergen Gross <jgross@suse.com> # v1 > Cc: Borislav Petkov <bp@alien8.de> > Cc: Dave Hansen <dave.hansen@intel.com> > Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com> > Cc: stable@vger.kernel.org # v3.19+ Reviewed-by: Juergen Gross <jgross@suse.com> Juergen
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h index 447d4bee25c48..97533e6b1c61b 100644 --- a/arch/x86/include/asm/pgtable_types.h +++ b/arch/x86/include/asm/pgtable_types.h @@ -125,11 +125,12 @@ * instance, and is *not* included in this mask since * pte_modify() does modify it. */ -#define _PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \ - _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY | \ - _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | _PAGE_ENC | \ - _PAGE_UFFD_WP) -#define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE) +#define _COMMON_PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \ + _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |\ + _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | _PAGE_ENC | \ + _PAGE_UFFD_WP) +#define _PAGE_CHG_MASK (_COMMON_PAGE_CHG_MASK | _PAGE_PAT) +#define _HPAGE_CHG_MASK (_COMMON_PAGE_CHG_MASK | _PAGE_PSE | _PAGE_PAT_LARGE) /* * The cache modes defined here are used to translate between pure SW usage