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

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Message-ID: <87seumt89u.fsf@redhat.com>
Date: Fri, 30 Aug 2024 16:06:21 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Gerd Hoffmann <kraxel@...hat.com>, 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

Sean Christopherson <seanjc@...gle.com> writes:

> On Fri, Aug 30, 2024, Vitaly Kuznetsov wrote:
>> 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.
>
> 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