Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Tue, 7 Feb 2023 09:49:24 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Juergen Gross <jgross@suse.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        lists@nerdbynature.de, mikelley@microsoft.com,
        torvalds@linux-foundation.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 2/6] x86/pat: check for MTRRs enabled in memtype_reserve()
Message-ID: <Y+IQlJI33snDiLT1@gmail.com>
References: <20230207072902.5528-1-jgross@suse.com>
 <20230207072902.5528-3-jgross@suse.com>
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* Juergen Gross <jgross@suse.com> wrote:

> Today memtype_reserve() bails out early if pat_enabled() returns false.
> The same can be done in case MTRRs aren't enabled.
> 
> This will reinstate the behavior of memtype_reserve() before commit
> 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when
> running on Xen"). There have been reports about that commit breaking
> SEV-SNP guests under Hyper-V, which was tried to be resolved by commit
> 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case"),
> but that again resulted in problems with Xen PV guests.
> 
> Fixes: 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when running on Xen")
> Fixes: 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case")
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/mm/pat/memtype.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> index fb4b1b5e0dea..18f612b43763 100644
> --- a/arch/x86/mm/pat/memtype.c
> +++ b/arch/x86/mm/pat/memtype.c
> @@ -557,8 +557,12 @@ int memtype_reserve(u64 start, u64 end, enum page_cache_mode req_type,
>  		return -EINVAL;
>  	}
>  
> -	if (!pat_enabled()) {
> -		/* This is identical to page table setting without PAT */
> +	/*
> +	 * PAT disabled or MTRRs disabled don't require any memory type
> +	 * tracking or type adjustments, as there can't be any conflicts
> +	 * between PAT and MTRRs with at least one of both being disabled.
> +	 */
> +	if (!pat_enabled() || !mtrr_enabled()) {
>  		if (new_type)
>  			*new_type = req_type;

Doesn't memtype_reserve() also check for overlapping ranges & type 
compatibility in memtype_check_conflict(), etc., which can occur even in a 
pure PAT setup? Ie. are we 100% sure that in the !MTRR case it would be a 
NOP?

But even if it's a functional NOP as you claim, we'd still be better off if 
the memtype tree was still intact - instead of just turning off the API.

Also, speling nit:

   s/one of both
    /one or both

Thanks,

	Ingo