Messages in this thread | | | Date | Mon, 24 Apr 2023 15:29:59 +0200 | From | Juergen Gross <> | Subject | Re: [RFC PATCH] x86/mm: Fix PAT bit missing from page protection modify mask |
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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. [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |