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[91.219.240.8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42ba639671dsm81832555e9.5.2024.08.30.07.06.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Aug 2024 07:06:22 -0700 (PDT) From: Vitaly Kuznetsov To: Sean Christopherson Cc: Gerd Hoffmann , Paolo Bonzini , 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 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> <871q26unq8.fsf@redhat.com> Date: Fri, 30 Aug 2024 16:06:21 +0200 Message-ID: <87seumt89u.fsf@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Sean Christopherson writes: > On Fri, Aug 30, 2024, Vitaly Kuznetsov wrote: >> Gerd Hoffmann writes: >> >> >> 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: >> > >> > qemu stdvga (the default display device) is affected too. >> > >> >> So far, I was only able to verify that the issue has nothing to do with >> OVMF and multi-vcpu, it reproduces very well with >> >> $ qemu-kvm -machine q35,accel=kvm,kernel-irqchip=split -name guest=c10s >> -cpu host -smp 1 -m 16384 -drive file=/var/lib/libvirt/images/c10s-bios.qcow2,if=none,id=drive-ide0-0-0 >> -device ide-hd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1 >> -vnc :0 -device VGA -monitor stdio --no-reboot >> >> Comparing traces of working and broken cases, I couldn't find anything >> suspicious but I may had missed something of course. For now, it seems >> like a userspace misbehavior resulting in a segfault. > > Guest userspace? > Yes? :-) As Gerd described, video memory is "mapped into userspace so the wayland / X11 display server can software-render into the buffer" and it seems that wayland gets something unexpected in this memory and crashes. -- Vitaly