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

[Maxim Levitsky]

On Thu, 2022-01-13 at 15:36 +0100, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes:
> 
> > On Thu, 2022-01-13 at 10:27 +0100, Vitaly Kuznetsov wrote:
> > > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> > > 
> > > > On 1/12/22 14:58, Vitaly Kuznetsov wrote:
> > > > > -	best = kvm_find_cpuid_entry(vcpu, 0xD, 1);
> > > > > +	best = cpuid_entry2_find(entries, nent, 0xD, 1);
> > > > >   	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
> > > > >   		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> > > > >   		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> > > > >   
> > > > > -	best = kvm_find_kvm_cpuid_features(vcpu);
> > > > > +	best = __kvm_find_kvm_cpuid_features(vcpu, vcpu->arch.cpuid_entries,
> > > > > +					     vcpu->arch.cpuid_nent);
> > > > >   	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> > > > 
> > > > I think this should be __kvm_find_kvm_cpuid_features(vcpu, entries, nent).
> > > > 
> > > 
> > > Of course.
> > > 
> > > > > +		case 0x1:
> > > > > +			/* Only initial LAPIC id is allowed to change */
> > > > > +			if (e->eax ^ best->eax || ((e->ebx ^ best->ebx) >> 24) ||
> > > > > +			    e->ecx ^ best->ecx || e->edx ^ best->edx)
> > > > > +				return -EINVAL;
> > > > > +			break;
> > > > 
> > > > This XOR is a bit weird.  In addition the EBX test is checking the wrong 
> > > > bits (it checks whether 31:24 change and ignores changes to 23:0).
> > > 
> > > Indeed, however, I've tested CPU hotplug with QEMU trying different
> > > CPUs in random order and surprisingly othing blew up, feels like QEMU
> > > was smart enough to re-use the right fd)
> > > 
> > > > You can write just "(e->ebx & ~0xff000000u) != (best->ebx ~0xff000000u)".
> > > > 
> > > > > +		default:
> > > > > +			if (e->eax ^ best->eax || e->ebx ^ best->ebx ||
> > > > > +			    e->ecx ^ best->ecx || e->edx ^ best->edx)
> > > > > +				return -EINVAL;
> > > > 
> > > > This one even more so.
> > > 
> > > Thanks for the early review, I'm going to prepare a selftest and send
> > > this out.
> > > 
> > I also looked at this recently (due to other reasons) and I found out that
> > qemu picks a parked vcpu by its vcpu_id which is its initial apic id,
> > thus apic id related features should not change.
> > 
> > Take a look at 'kvm_get_vcpu' in qemu source.
> > Maybe old qemu versions didn't do this?
> 
> I took Igor's word on this, I didn't check QEMU code :-)
> 
> In the v1 I've just sent [L,x2]APIC ids are allowed to change. This
> shouldn't screw the MMU (which was the main motivation for forbidding
> KVM_SET_CPUID{,2} after KVM_RUN in the first place) but maybe we don't
> really need to be so permissive.
> 

For my nested AVIC work I would really want the APIC ID of a VCPU to be read-only
and be equal to vcpu_id.

That simplifies lot of things, and in practice it is hightly likely that no guests
change their APIC id, and likely that qemu doesn't as well.

Best regards,
	Maxim Levitsky