Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT without MTRR

[Juergen Gross <jgross@xxxxxxxx>]

On 13.07.22 03:36, Chuck Zmudzinski wrote:
> The commit 99c13b8c8896d7bcb92753bf
> ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> incorrectly failed to account for the case in init_cache_modes() when
> CPUs do support PAT and falsely reported PAT to be disabled when in
> fact PAT is enabled. In some environments, notably in Xen PV domains,
> MTRR is disabled but PAT is still enabled, and that is the case
> that the aforementioned commit failed to account for.
>
> As an unfortunate consequnce, the pat_enabled() function currently does
> not correctly report that PAT is enabled in such environments. The fix
> is implemented in init_cache_modes() by setting pat_bp_enabled to true
> in init_cache_modes() for the case that commit 99c13b8c8896d7bcb92753bf
> ("x86/mm/pat: Don't report PAT on CPUs that don't support it") failed
> to account for.
>
> This approach arranges for pat_enabled() to return true in the Xen PV
> environment without undermining the rest of PAT MSR management logic
> that considers PAT to be disabled: Specifically, no writes to the PAT
> MSR should occur.
>
> This patch fixes a regression that some users are experiencing with
> Linux as a Xen Dom0 driving particular Intel graphics devices by
> correctly reporting to the Intel i915 driver that PAT is enabled where
> previously it was falsely reporting that PAT is disabled. Some users
> are experiencing system hangs in Xen PV Dom0 and all users on Xen PV
> Dom0 are experiencing reduced graphics performance because the keying of
> the use of WC mappings to pat_enabled() (see arch_can_pci_mmap_wc())
> means that in particular graphics frame buffer accesses are quite a bit
> less performant than possible without this patch.
>
> Also, with the current code, in the Xen PV environment, PAT will not be
> disabled if the administrator sets the "nopat" boot option. Introduce
> a new boolean variable, pat_force_disable, to forcibly disable PAT
> when the administrator sets the "nopat" option to override the default
> behavior of using the PAT configuration that Xen has provided.
>
> For the new boolean to live in .init.data, init_cache_modes() also needs
> moving to .init.text (where it could/should have lived already before).
>
> Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't 
> support it")
> Co-developed-by: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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