linux-kernel - Re: [PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen

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Date:   Tue, 5 Jul 2022 17:04:32 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [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.

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