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

[Chuck Zmudzinski <brchuckz@xxxxxxxxxxxx>]

On 7/12/22 3:18 PM, Chuck Zmudzinski wrote:
> 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

I will also re-compile and test the new patch before sending
v2 and unless Jan objects, I should acknowledge Jan as co-author
of the patch since I will be using parts of his proposed patch
to fix the "nopat" issue, so I also need to get his sign-off before
sending v2. Jan, how should I obtain your sign-off?

Chuck

Chuck