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

[Maxim Levitsky]

On Thu, 2022-01-13 at 22:33 +0000, Sean Christopherson wrote:
> On Mon, Jan 03, 2022, Igor Mammedov wrote:
> > 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.
> 
> 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.

So after all, it is true that vcpu_id == initial APIC_ID,
and if we don't let guest change it, it will be always like that? 


You said that its not true in the other mail in the thread. 
I haven't checked it in the code yet, as I never was much worried about userspace changing,
but I will check it soon.

I did a quick look and I see that at least the userspace can call 'kvm_apic_set_state' and it 
contains snapshot of all apic registers, including apic id.
However it would be very easy to add a check
there and fail if userspace attempts to
set APIC_ID != vcpu_id.


Best regards,
	Maxim Levitsky
> 
> 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.
> 
>   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);
>   }
>