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
