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

From: Borislav Petkov <bp@alien8.de>
To: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Lutomirski <luto@kernel.org>,
	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	lkml <linux-kernel@vger.kernel.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen
Date: Tue, 5 Jul 2022 17:04:32 +0200	[thread overview]
Message-ID: <YsRTAGI2PhfZ5V7M@zn.tnic> (raw)
In-Reply-To: <9385fa60-fa5d-f559-a137-6608408f88b0@suse.com>

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
