Igor Mammedov <imammedo@xxxxxxxxxx> writes: > On Mon, 03 Jan 2022 13:56:53 +0100 > Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: > >> 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. > > Can you post a patch, then? > Paolo, would you like to send a last minute revert to Linus to save 5.16 ... >> 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. > > It might work, at least on QEMU side we do not allow mixing up CPU models > within VM instance (so far). So aside of APICid (and related leafs > (extended APIC ID/possibly other topo related stuff)) nothing else should > change ever when a new vCPU is hotplugged. or should we just focus on this (or similar) solution (for 5.17 and stable@5.16)? > >> 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. > In QEMU, 'parked cpus' fd list is a generic code shared by all supported > archs. And I'm reluctant to push something x86 specific there (it's not > impossible, but it's a crutch to workaround forbidden KVM_SET_CPUID{,2}). > You can see it this was: vCPU fd corresponds to the particular CPU being hot plugged/unplugged, not to any CPU in the guest system. The change may be generic enough then (but it's not going to save existing QEMUs of course). -- Vitaly
