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

[Chuck Zmudzinski <brchuckz@xxxxxxxxxxxx>]

On 7/14/2022 1:30 AM, Thorsten Leemhuis wrote:
> 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")
>
> BTW, "submitting-patches.rst" says it should just be "the first 12
> characters of the SHA-1 ID"

Actually it says "at least", so more that 12 is It is probably safest
to put the entire SHA-1 ID in because of the possibility of
a collision. At least that's how I understand what
submitting-patches.rst.

>
> > Co-developed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
>
> Sorry, have to ask: is this supposed to finally fix this regression?
> https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/

Yes that's the first report I saw to lkml about this isssue. So if I submit
a v3 I will include that. But my patch does not have a sign-off from the
Co-developer as I mentioned in a message earlier today, and the
discussion that has ensued leads me to think a better solution is to just
revert the commit in the i915 driver instead, and leave the bigger questions
for Juergen Gross and his plans to re-work the x86/PAT code in September,
as he said on this thread in the last couple of days. This patch is dead
now,
as far as I can tell, because the Co-developer is not cooperating.

Chuck
>
> If yes, please include Link: and Reported-by: tags, as explained in
> submitting-patches.rst (I only care about the link tag, as I'm tacking
> that regression).
>
> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
>
> P.S.: As the Linux kernel's regression tracker I deal with a lot of
> reports and sometimes miss something important when writing mails like
> this. If that's the case here, don't hesitate to tell me in a public
> reply, it's in everyone's interest to set the public record straight.