[RFC PATCH] x86/mm: Fix PAT bit missing from page protection modify mask

[Andi Shyti]

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 at linux.intel.com>
> Cc: stable at 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 at 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