Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

[Paolo Bonzini]

On 12/27/21 18:32, Igor Mammedov wrote:
> > Tweaked and queued nevertheless, thanks.
> it seems this patch breaks VCPU hotplug, in scenario:
>
>    1. hotunplug existing VCPU (QEMU stores VCPU file descriptor in parked cpus list)
>    2. hotplug it again (unsuspecting QEMU reuses stored file descriptor when recreating VCPU)
>
> RHBZ:https://bugzilla.redhat.com/show_bug.cgi?id=2028337#c11

The fix here would be (in QEMU) to not call KVM_SET_CPUID2 again. 
However, we need to work around it in KVM, and allow KVM_SET_CPUID2 if 
the data passed to the ioctl is the same that was set before.

Paolo