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

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 13 Jan 2022 22:33:38 +0000
Sean Christopherson <seanjc@google.com> wrote:

> On Mon, Jan 03, 2022, Igor Mammedov wrote:
> > 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.  
> 
> No, it cannot.  KVM doesn't provide a way for userspace to change the APIC ID of
> a vCPU after the vCPU is created.  x2APIC flat out disallows changing the APIC ID,
> and unless there's magic I'm missing, apic_mmio_write() => kvm_lapic_reg_write()
> is not reachable from userspace.
> 
> The only way for userspace to set the APIC ID is to change vcpu->vcpu_id, and that
> can only be done at KVM_VCPU_CREATE.
> 
> So, reusing a parked vCPU for hotplug must reuse the same APIC ID.  QEMU handles
> this by stashing the vcpu_id, a.k.a. APIC ID, when parking a vCPU, and reuses a
> parked vCPU if and only if it has the same APIC ID.  And because QEMU derives the
> APIC ID from topology, that means all the topology CPUID leafs must remain the
> same, otherwise the guest is hosed because it will send IPIs to the wrong vCPUs.

Indeed, I was wrong.
I just checked all cpu unplug history in qemu. It was introduced in qemu-2.7
and from the very beginning it did stash vcpu_id,
so there is no old QEMU that would re-plug VCPU with different apic_id.
Though tells us nothing about what other userspace implementations might do.

However, a problem of failing KVM_SET_CPUID2 during VCPU re-plug
is still there and re-plug will fail if KVM rejects repeated KVM_SET_CPUID2
even if ioctl called with exactly the same CPUID leafs as the 1st call.


>   static int do_kvm_destroy_vcpu(CPUState *cpu)
>   {
>     struct KVMParkedVcpu *vcpu = NULL;
> 
>     ...
> 
>     vcpu = g_malloc0(sizeof(*vcpu));
>     vcpu->vcpu_id = kvm_arch_vcpu_id(cpu); <=== stash the APIC ID when parking
>     vcpu->kvm_fd = cpu->kvm_fd;
>     QLIST_INSERT_HEAD(&kvm_state->kvm_parked_vcpus, vcpu, node);
> err:
>     return ret;
>   }
> 
>   static int kvm_get_vcpu(KVMState *s, unsigned long vcpu_id)
>   {
>     struct KVMParkedVcpu *cpu;
> 
>     QLIST_FOREACH(cpu, &s->kvm_parked_vcpus, node) {
>         if (cpu->vcpu_id == vcpu_id) {  <=== reuse if APIC ID matches
>             int kvm_fd;
> 
>             QLIST_REMOVE(cpu, node);
>             kvm_fd = cpu->kvm_fd;
>             g_free(cpu);
>             return kvm_fd;
>         }
>     }
> 
>     return kvm_vm_ioctl(s, KVM_CREATE_VCPU, (void *)vcpu_id);
>   }
>