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

[Yan Zhao <yan.y.zhao@intel.com>]

On Mon, Sep 02, 2024 at 11:49:43AM +0200, Vitaly Kuznetsov wrote:
> Yan Zhao <yan.y.zhao@intel.com> writes:
> 
> > On Fri, Aug 30, 2024 at 03:47:11PM +0200, Vitaly Kuznetsov wrote:
> >> Gerd Hoffmann <kraxel@redhat.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.
Hmm, looks this issue can't be reproduced on physical machine "Coffee Lake-S".
The installer can show up to ask for language selection.

But it can be reproduced on the machine "Sapphire Rapids XCC".

> 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.
>
Same to the gdm.

> 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".
Will have a debug to check what's going wrong. Thanks!