On Thu, 28 Dec 2023 at 17:21, <x...@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. >
