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    SubjectRe: [PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen
    On Thu, Apr 28, 2022 at 04:50:29PM +0200, Jan Beulich wrote:
    > --- a/arch/x86/mm/pat/memtype.c
    > +++ b/arch/x86/mm/pat/memtype.c
    > @@ -62,6 +62,7 @@
    >
    > static bool __read_mostly pat_bp_initialized;
    > static bool __read_mostly pat_disabled = !IS_ENABLED(CONFIG_X86_PAT);
    > +static bool __initdata pat_force_disabled = !IS_ENABLED(CONFIG_X86_PAT);
    > static bool __read_mostly pat_bp_enabled;
    > static bool __read_mostly pat_cm_initialized;

    Why yet another boolean var?

    Why not extend pat_enabled() to reflect the Xen case and explain it
    properly above it?

    My comment is likely wrong because I don't know what the Xen HV hides or
    doesn't but you get the idea...

    diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
    index d5ef64ddd35e..a8f1a02f9bc2 100644
    --- a/arch/x86/mm/pat/memtype.c
    +++ b/arch/x86/mm/pat/memtype.c
    @@ -92,6 +92,13 @@ early_param("nopat", nopat);

    bool pat_enabled(void)
    {
    + /*
    + * Xen PV doesn't expose the PAT MSR to dom0 so the proper init path
    + * there cannot be exercised. Announce PAT is enabled in that case too.
    + */
    + if (cpu_feature_enabled(X86_FEATURE_XENPV) && !pat_disabled)
    + return true;
    +
    return pat_bp_enabled;
    }
    EXPORT_SYMBOL_GPL(pat_enabled);
    --
    Regards/Gruss,
    Boris.

    https://people.kernel.org/tglx/notes-about-netiquette

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