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

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Date:   Tue, 12 Jul 2022 15:48:25 -0400
From:   Chuck Zmudzinski <brchuckz@...scape.net>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        Jan Beulich <jbeulich@...e.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Jane Chu <jane.chu@...cle.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        xen-devel@...ts.xenproject.org, stable@...r.kernel.org
Subject: Re: [PATCH] x86/PAT: Report PAT on CPUs that support PAT without MTRR



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@...r.kernel.org
> > > Signed-off-by: Chuck Zmudzinski <brchuckz@....com>
> > > ---
> > > 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