Igor Mammedov <imammedo@xxxxxxxxxx> writes: > On Mon, 03 Jan 2022 09:04:29 +0100 > Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: > >> Paolo Bonzini <pbonzini@xxxxxxxxxx> writes: >> >> > 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. >> >> Are we sure the data is going to be *exactly* the same? In particular, >> when using vCPU fds from the parked list, do we keep the same >> APIC/x2APIC id when hotplugging? Or can we actually hotplug with a >> different id? > > If I recall it right, it can be a different ID easily. > It's broken then. I'd suggest we revert the patch from KVM and think about the strategy how to proceed. Going forward, we really want to ban KVM_SET_CPUID{,2} after KVM_RUN (see the comment which my patch moves). E.g. we can have an 'allowlist' of things which can change (and put *APICids there) and only fail KVM_SET_CPUID{,2} when we see something else changing. In QEMU, we can search the parked CPUs list for an entry with the right *APICid and reuse it only if we manage to find one. -- Vitaly
