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

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Message-ID: <87jzfutmfc.fsf@redhat.com>
Date: Mon, 02 Sep 2024 11:49:43 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Sean Christopherson <seanjc@...gle.com>, 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>, 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

Yan Zhao <yan.y.zhao@...el.com> writes:

> On Fri, Aug 30, 2024 at 03:47:11PM +0200, 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.
> Could you please share steps launch the broken guest desktop?
> (better also with guest kernel version, name of desktop processes,
>  name of X server)

I think the easiest would be to download the latest Centos Stream 10
iso, e.g:

https://composes.stream.centos.org/stream-10/development/CentOS-Stream-10-20240902.d.0/compose/BaseOS/x86_64/iso/CentOS-Stream-10-20240902.d.0-x86_64-dvd1.iso
(the link is probably not eternal but should work for a couple weeks,
check https://composes.stream.centos.org/stream-10/development/ it it
doesn't work anymore).

Then, just run it:
$ /usr/libexec/qemu-kvm -machine q35,accel=kvm,kernel-irqchip=split -name guest=c10s -cpu host -smp 1 -m 16384 -cdrom CentOS-Stream-10-20240902.d.0-x86_64-dvd1.iso -vnc :0 -device VGA -monitor stdio --no-reboot

and connect to VNC console. To speed things up, pick 'Install Centos
Stream 10' in the boot menu to avoid ISO integrity check.

With "KVM: VMX: Always honor guest PAT on CPUs that support self-snoop"
commit included, you will see the following on the VNC console:
installer tries starting Wayland, crashes and drops back into text
console. If you revert the commit and start over, Wayland will normally
start and you will see the installer.

If the installer environment is inconvenient for debugging, then you can
install in text mode (or with the commit reverted :-) and then the same
problem will be observed when gdm starts.

FWIW, I use QEMU-9.0 from the same C10S (qemu-kvm-9.0.0-7.el10.x86_64)
but I don't think it matters in this case. My CPU is "Intel(R) Xeon(R)
Silver 4410Y".

-- 
Vitaly