Re: Qemu setting "-cpu host" seems broken with Windows vms

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, 28 Dec 2023 at 17:21, <xtec@trimaso.com.mx> wrote:

CCing Paolo, the general x86 maintainer.

Stefan

> I noticed something weird when using "-cpu host" with Windows vms.
> First, I always use it along with ",hv_passthrough" as well.
>
> First, performance: since some years ago, since prior to qemu 6.2 until
> latest 8.2, win10 and win11 vms always worked slower than expected. This
> could be noticed by comparing booting/starting times between vm and a
> bare metal installation, but I particularly measured it when installing
> windows cumulative updates through windows update. On vm, from
> downloading to finishing rebooting it always took 1.5 circa 1.5 hours,
> while just 40 minutes on bare metal.
>
> Second, and more recently, newer windows 11 23h2 seems to have big
> problem with "-cpu host".
> When trying to update from 22h2 to 23h2 I got either black screen or
> bsod after trying to reboot.
> Also, same result when trying to install 23h2 from scratch.
> This on qemu 7.1 and 8.2.
> Did a long search, and finally found the cause which also solved the
> problem for me:
> https://forum.proxmox.com/threads/new-windows-11-vm-fails-boot-after-update.137543/
> I found similar problems and similar solution in other forums as well.
>
> So in my case, physical host cpu is intel core 11th gen; tried using
> libvirt's "virsh capabilities" to see which qemu cpu model better
> matched, and for some reason it gave Broadwell instead of newer
> Skylake...
> Anyway, tried with "-cpu <Broadwell_model>,hv_passthrough", and this
> solved *both* problems: performance finally matched bare metal in all
> aspects, and the windows 23h2 problem was finally gone.
>
> On IRC, it was suggested to try "-cpu host" and "disabling CPU bits" one
> by one until finding the culprit. But I don't know how to do this...
>
> Could someone look into this?
> Thanks.
>