linux-kernel - Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM

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Date:   Thu, 13 Jan 2022 15:59:59 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Igor Mammedov <imammedo@...hat.com>
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

Maxim Levitsky <mlevitsk@...hat.com> writes:

> On Thu, 2022-01-13 at 15:36 +0100, Vitaly Kuznetsov wrote:
>> Maxim Levitsky <mlevitsk@...hat.com> writes:
>> 
>> > On Thu, 2022-01-13 at 10:27 +0100, Vitaly Kuznetsov wrote:
>> > > Paolo Bonzini <pbonzini@...hat.com> 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.
>

Doesn't APIC ID have topology encoded in it?

> 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.

-- 
Vitaly