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 17:56:36 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Borislav Petkov <bp@...en8.de>
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 05.07.2022 17:04, Borislav Petkov wrote:
> 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?

Re-using pat_disabled like you do in your suggestion below won't
work, because mtrr_bp_init() calls pat_disable() when MTRRs
appear to be disabled (from the kernel's view). The goal is to
honor "nopat" without honoring any other calls to pat_disable().

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

I can probably fiddle with pat_enabled() instead of with
init_cache_modes(), but when making the change I had the feeling
this might be less liked (as looking more hacky, at least to me).

But besides the "where" the other question is: Do you really want
me to limit this to Xen/PV, rather than - as I have it now -
extending it to any hypervisor, which may behave in similar ways?

Jan

> 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);
>