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

[Chuck Zmudzinski]

On 7/12/22 2:27 PM, Juergen Gross wrote:
> On 12.07.22 20:20, 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
> > ---
> > Reminder: This patch is a regression fix that is needed on stable
> > versions 5.17 and later.
> > 
> >   arch/x86/mm/pat/memtype.c | 12 ++++++++++++
> >   1 file changed, 12 insertions(+)
> > 
> > diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> > index d5ef64ddd35e..0f2417bd1b40 100644
> > --- a/arch/x86/mm/pat/memtype.c
> > +++ b/arch/x86/mm/pat/memtype.c
> > @@ -315,6 +315,18 @@ void init_cache_modes(void)
> >   		      PAT(4, WB) | PAT(5, WT) | PAT(6, UC_MINUS) | PAT(7, UC);
> >   	}
> >   
> > +	else if (!pat_bp_enabled) {
>
> Please put the "else if {" into the same line as the "}" above.
>
> > +	/*
> > +	 * In some environments, specifically Xen PV, PAT
> > +	 * initialization is skipped because MTRRs are disabled even
> > +	 * though PAT is available. In such environments, set PAT to
> > +	 * enabled to correctly indicate to callers of pat_enabled()
> > +	 * that CPU support for PAT is available.
> > +	 */
> > +	pat_bp_enabled = true;
> > +	pr_info("x86/PAT: PAT enabled by init_cache_modes\n");
>
> Wrong indentation.
>
> > +	}
> > +
> >   	__init_cache_modes(pat);
> >   }
> >   
>
> Any reason you didn't fix the "nopat" issue Jan mentioned?
>
> I asked you twice to add this fix.
>
>
> Juergen

Sorry, I did not see your request. I will resend with the fix
for "nopat" and the other style issues you mentioned.

Chuck