Re: [PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen

[Juergen Gross <jgross@suse.com>]

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On 28.04.22 16:50, Jan Beulich wrote:
> The latest with commit bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT
> with pat_enabled()") pat_enabled() returning false (because of PAT
> initialization being suppressed in the absence of MTRRs being announced
> to be available) has become a problem: The i915 driver now fails to
> initialize when running PV on Xen (i915_gem_object_pin_map() is where I
> located the induced failure), and its error handling is flaky enough to
> (at least sometimes) result in a hung system.
> 
> Yet even beyond that problem 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 would have been quite a bit less
> performant than possible.
> 
> Arrange for the function to return true in such environments, without
> undermining the rest of PAT MSR management logic considering PAT to be
> disabled: Specifically, no writes to the PAT MSR should occur.
> 
> 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).
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

I think this approach isn't the best way to tackle the issue.

It can be solved rather easily by not deriving the supported caching
modes via pat_enabled(), but by adding specific functions to query
the needed caching mode from the PAT translation tables, and to use
those functions instead of pat_enabled().

I'm preparing a patch for that purpose.


Juergen

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