linux-kernel - Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop

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Message-ID: <871q26unq8.fsf@redhat.com>
Date: Fri, 30 Aug 2024 15:47:11 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Gerd Hoffmann <kraxel@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 rcu@...r.kernel.org, linux-kernel@...r.kernel.org, Kevin Tian
 <kevin.tian@...el.com>, Yan Zhao <yan.y.zhao@...el.com>, Yiwei Zhang
 <zzyiwei@...gle.com>, Lai Jiangshan <jiangshanlai@...il.com>, "Paul E.
 McKenney" <paulmck@...nel.org>, Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that
 support self-snoop

Gerd Hoffmann <kraxel@...hat.com> 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.

-- 
Vitaly