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Re: [PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen
On 03.05.22 14:54, Juergen Gross wrote:
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@xxxxxxxx>
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.
That attempt was not a complete success.
Especially there are issues with my approach when "nopat" has been
specified as boot parameter, as that would be just ignored.
So right now I can't think of a better approach than the one of Jan's
patch.
Juergen
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