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

[Paolo Bonzini]

On Fri, Dec 29, 2023 at 2:10 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > 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.

One possibility is that you have Hyper-V enabled with -cpu host but
not with other CPU models. That's because "-cpu host" enables nested
virtualization.

Try "-cpu host,-vmx" and it should be clear if that's the case.

Based on the pastie that you prepared, that's the main difference
between -cpu host and -cpu Broadwell-noTSX-IBRS. Nothing else (see
list below) should have any substantial performance impact; even less
so should they make things worse.

Paolo

               "avx512-vp2intersect": true,
               "avx512-vpopcntdq": true,
               "avx512bitalg": true,
               "avx512bw": true,
               "avx512cd": true,
               "avx512dq": true,
               "avx512f": true,
               "avx512ifma": true,
               "avx512vbmi": true,
               "avx512vbmi2": true,
               "avx512vl": true,
               "avx512vnni": true,
               "full-width-write": true,
               "gfni": true,
               "vaes": true,
               "vpclmulqdq": true,

               "clflushopt": true,
               "clwb": true,

               "fsrm": true,

               "host-cache-info": false,
               "host-phys-bits": true,

               "amd-ssbd": true,
               "amd-stibp": true,
               "arch-capabilities": true,
               "ibpb": true,
               "ibrs": true,
               "ibrs-all": true,
               "ssbd": true,
               "stibp": true,

               "kvm-pv-ipi": true,
               "kvm-pv-sched-yield": true,
               "kvm-pv-tlb-flush": true,
               "kvm-pv-unhalt": true,

               "lmce": true,
               "md-clear": true,
               "mds-no": true,
               "movdir64b": true,
               "movdiri": true,
               "pdcm": true,
               "pdpe1gb": true,

               "pdcm": false,
               "pdpe1gb": false,
               "pku": true,
               "pmu": true,
               "pschange-mc-no": true,
               "rdctl-no": true,
               "rdpid": true,
               "sha-ni": true,
               "ss": true,
               "tsc-adjust": true,
               "umip": true,
               "vmx": true,
               "xgetbv1": true,
               "xsavec": true,
               "xsaves": true,

(skipped everything vmx-related, since they don't matter with vmx
itself being false)