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

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jul 12, 2022 at 03:16:01PM -0400, Chuck Zmudzinski wrote:
> On 7/12/22 2:36 PM, Greg KH wrote:
> > On Tue, Jul 12, 2022 at 02:20:37PM -0400, 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 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.
> > > 
> > > Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
> > > ---
> > > Reminder: This patch is a regression fix that is needed on stable
> > > versions 5.17 and later.
> >
> > Then why are you saying it fixes a commit that is in 4.4.y and newer?
> >
> > confused,
> >
> > greg k-h
> 
> It is true the erroneous reporting of PAT goes back to 4.4.y. But it
> was not until 5.17.y when the i915 driver was patched with a commit
> that started using pat_enabled() instead of boot_cpu_has(X86_FEATURE_PAT)
> and that is when a regression that started annoying users appeared
> in the kernel. I presume that we only backport patches to stable that
> fix regressions that are really bothering users, so even though the
> problem dates to 4.4.y, there is no need to backport before 5.17.y
> which is when the problem manifested in a way that started
> bothering users.

If it needs to go back to 4.9.y or so, let's take it all the way back to
be consistent everywhere.

thanks,

greg k-h