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[91.219.240.8]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37d1690f75bsm5766858f8f.23.2024.10.07.06.38.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 07 Oct 2024 06:38:16 -0700 (PDT) From: Vitaly Kuznetsov To: "Linux regression tracking (Thorsten Leemhuis)" , Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, Kevin Tian , Yan Zhao , Yiwei Zhang , Lai Jiangshan , "Paul E. McKenney" , Josh Triplett , Gerd Hoffmann , Linux kernel regressions list Subject: Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop In-Reply-To: References: <20240309010929.1403984-1-seanjc@google.com> <20240309010929.1403984-6-seanjc@google.com> <877cbyuzdn.fsf@redhat.com> Date: Mon, 07 Oct 2024 15:38:15 +0200 Message-ID: <87iku4ghiw.fsf@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain "Linux regression tracking (Thorsten Leemhuis)" writes: > On 30.08.24 11:35, Vitaly Kuznetsov wrote: >> Sean Christopherson writes: >> >>> Unconditionally honor guest PAT on CPUs that support self-snoop, as >>> Intel has confirmed that CPUs that support self-snoop always snoop caches >>> and store buffers. I.e. CPUs with self-snoop maintain cache coherency >>> even in the presence of aliased memtypes, thus there is no need to trust >>> the guest behaves and only honor PAT as a last resort, as KVM does today. >>> >>> Honoring guest PAT is desirable for use cases where the guest has access >>> to non-coherent DMA _without_ bouncing through VFIO, e.g. when a virtual >>> (mediated, for all intents and purposes) GPU is exposed to the guest, along >>> with buffers that are consumed directly by the physical GPU, i.e. which >>> can't be proxied by the host to ensure writes from the guest are performed >>> with the correct memory type for the GPU. >> >> Necroposting! >> >> Turns out that this change broke "bochs-display" driver in QEMU even >> when the guest is modern (don't ask me 'who the hell uses bochs for >> modern guests', it was basically a configuration error :-). E.g: >> [...] > > This regression made it to the list of tracked regressions. It seems > this thread stalled a while ago. Was this ever fixed? Does not look like > it, but I might have missed something. Or is this a regression I should > just ignore for one reason or another? > The regression was addressed in by reverting 377b2f359d1f in 6.11 commit 9d70f3fec14421e793ffbc0ec2f739b24e534900 Author: Paolo Bonzini Date: Sun Sep 15 02:49:33 2024 -0400 Revert "KVM: VMX: Always honor guest PAT on CPUs that support self-snoop" Also, there's a (pending) DRM patch fixing it from the guest's side: https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/9388ccf69925223223c87355a417ba39b13a5e8e -- Vitaly