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

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 03 Jan 2022 13:56:53 +0100
Vitaly Kuznetsov <vkuznets@redhat.com> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
> 
> > On Mon, 03 Jan 2022 09:04:29 +0100
> > Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >  
> >> Paolo Bonzini <pbonzini@redhat.com> 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?

> 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.

> 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}).