| Message ID | 20230424123524.17008-1-janusz.krzysztofik@linux.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] x86/mm: Fix PAT bit missing from page protection modify mask | expand |
On 24.04.23 14:35, 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 test variants that > mmap the objects using different methods and caching modes. > > 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"). > > Extend bitmask used by pgprot_modify() for selecting bits to be preserved > with _PAGE_PAT bit. However, since that bit can be reused as _PAGE_PSE, > and the _PAGE_CHG_MASK symbol, primarly used by pte_modify(), is likely > intentionally defined with that bit not set, keep that symbol unchanged. Hmm, I wonder whether pte_mkhuge() shouldn't just set _PAGE_PSE, but use pgprot_4k_2_large() before doing so. OTOH a use case like in remove_migration_pte(), where pte_mkhuge() is directly followed by a call of arch_make_huge_pte(), which in turn is calling pte_mkhuge() again, would set _always_ the PAT bit. When running as a Xen PV guest this doesn't matter at all, as large or huge pages aren't supported there. So clearly something for the MM maintainers. :-) Juergen P.S.: Janusz, nice catch! The QubesOS folks who reported the problem originally will test your patch under Xen soon.
On Mon, Apr 24, 2023 at 02:35:24PM +0200, 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 test variants that > mmap the objects using different methods and caching modes. > > 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"). > > Extend bitmask used by pgprot_modify() for selecting bits to be preserved > with _PAGE_PAT bit. However, since that bit can be reused as _PAGE_PSE, > and the _PAGE_CHG_MASK symbol, primarly used by pte_modify(), is likely > intentionally defined with that bit not set, keep that symbol unchanged. > > 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> > Cc: stable@vger.kernel.org # v3.19+ I can confirm it fixes the issue, thanks! Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Hi Janusz, On Mon, Apr 24, 2023 at 02:35:24PM +0200, 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 test variants that > mmap the objects using different methods and caching modes. > > 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"). > > Extend bitmask used by pgprot_modify() for selecting bits to be preserved > with _PAGE_PAT bit. However, since that bit can be reused as _PAGE_PSE, > and the _PAGE_CHG_MASK symbol, primarly used by pte_modify(), is likely > intentionally defined with that bit not set, keep that symbol unchanged. > > 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> > Cc: stable@vger.kernel.org # v3.19+ > --- > arch/x86/include/asm/pgtable.h | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h > index 7425f32e52932..f797f8da2e5b6 100644 > --- a/arch/x86/include/asm/pgtable.h > +++ b/arch/x86/include/asm/pgtable.h > @@ -654,8 +654,10 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot) > #define pgprot_modify pgprot_modify > static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot) > { > - pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK; > - pgprotval_t addbits = pgprot_val(newprot) & ~_PAGE_CHG_MASK; > + unsigned long mask = _PAGE_CHG_MASK | _PAGE_CACHE_MASK; nice catch! Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com> Thanks, Andi > + > + pgprotval_t preservebits = pgprot_val(oldprot) & mask; > + pgprotval_t addbits = pgprot_val(newprot) & ~mask; > return __pgprot(preservebits | addbits); > } > > -- > 2.40.0
Hi Janusz, On Mon, Apr 24, 2023 at 02:35:24PM +0200, 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 test variants that > mmap the objects using different methods and caching modes. BTW, are you going to update those tests in IGT? Andi
Hi, a kind reminder on this patch, would be fantastic if anyone from the x86 maintainers cha give it a look. The patch has been tested thoroughly and even if it's marked as an RFC in my opinion it can be already considered for a proper review. Thanks, Andi On Mon, Apr 24, 2023 at 02:35:24PM +0200, 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 test variants that > mmap the objects using different methods and caching modes. > > 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"). > > Extend bitmask used by pgprot_modify() for selecting bits to be preserved > with _PAGE_PAT bit. However, since that bit can be reused as _PAGE_PSE, > and the _PAGE_CHG_MASK symbol, primarly used by pte_modify(), is likely > intentionally defined with that bit not set, keep that symbol unchanged. > > 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> > Cc: stable@vger.kernel.org # v3.19+ > --- > arch/x86/include/asm/pgtable.h | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h > index 7425f32e52932..f797f8da2e5b6 100644 > --- a/arch/x86/include/asm/pgtable.h > +++ b/arch/x86/include/asm/pgtable.h > @@ -654,8 +654,10 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot) > #define pgprot_modify pgprot_modify > static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot) > { > - pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK; > - pgprotval_t addbits = pgprot_val(newprot) & ~_PAGE_CHG_MASK; > + unsigned long mask = _PAGE_CHG_MASK | _PAGE_CACHE_MASK; > + > + pgprotval_t preservebits = pgprot_val(oldprot) & mask; > + pgprotval_t addbits = pgprot_val(newprot) & ~mask; > return __pgprot(preservebits | addbits); > } > > -- > 2.40.0
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index 7425f32e52932..f797f8da2e5b6 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -654,8 +654,10 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot) #define pgprot_modify pgprot_modify static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot) { - pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK; - pgprotval_t addbits = pgprot_val(newprot) & ~_PAGE_CHG_MASK; + unsigned long mask = _PAGE_CHG_MASK | _PAGE_CACHE_MASK; + + pgprotval_t preservebits = pgprot_val(oldprot) & mask; + pgprotval_t addbits = pgprot_val(newprot) & ~mask; return __pgprot(preservebits | addbits); }
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 test variants that mmap the objects using different methods and caching modes. 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"). Extend bitmask used by pgprot_modify() for selecting bits to be preserved with _PAGE_PAT bit. However, since that bit can be reused as _PAGE_PSE, and the _PAGE_CHG_MASK symbol, primarly used by pte_modify(), is likely intentionally defined with that bit not set, keep that symbol unchanged. 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> Cc: stable@vger.kernel.org # v3.19+ --- arch/x86/include/asm/pgtable.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)