Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 12 Jul 2022 20:36:30 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Chuck Zmudzinski <brchuckz@aol.com>
Cc:     linux-kernel@vger.kernel.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>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Jane Chu <jane.chu@oracle.com>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Sean Christopherson <seanjc@google.com>,
        Jan Beulich <jbeulich@suse.com>,
        Juergen Gross <jgross@suse.com>,
        xen-devel@lists.xenproject.org, stable@vger.kernel.org
Subject: Re: [PATCH] x86/PAT: Report PAT on CPUs that support PAT without MTRR
Message-ID: <Ys2/Lho9vQO33RZc@kroah.com>
References: <2885cdcaccffd287ef69c7509056ddf183a38a0e.1657647656.git.brchuckz.ref@aol.com>
 <2885cdcaccffd287ef69c7509056ddf183a38a0e.1657647656.git.brchuckz@aol.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <2885cdcaccffd287ef69c7509056ddf183a38a0e.1657647656.git.brchuckz@aol.com>
Precedence: bulk

On Tue, Jul 12, 2022 at 02:20:37PM -0400, Chuck Zmudzinski wrote:
> The commit 99c13b8c8896d7bcb92753bf
> ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> incorrectly failed to account for the case in init_cache_modes() when
> CPUs do support PAT and falsely reported PAT to be disabled when in
> fact PAT is enabled. In some environments, notably in Xen PV domains,
> MTRR is disabled but PAT is still enabled, and that is the case
> that the aforementioned commit failed to account for.
> 
> As an unfortunate consequnce, the pat_enabled() function currently does
> not correctly report that PAT is enabled in such environments. The fix
> is implemented in init_cache_modes() by setting pat_bp_enabled to true
> in init_cache_modes() for the case that commit 99c13b8c8896d7bcb92753bf
> ("x86/mm/pat: Don't report PAT on CPUs that don't support it") failed
> to account for.
> 
> This patch fixes a regression that some users are experiencing with
> Linux as a Xen Dom0 driving particular Intel graphics devices by
> correctly reporting to the Intel i915 driver that PAT is enabled where
> previously it was falsely reporting that PAT is disabled.
> 
> Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't support it")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
> ---
> Reminder: This patch is a regression fix that is needed on stable
> versions 5.17 and later.

Then why are you saying it fixes a commit that is in 4.4.y and newer?

confused,

greg k-h