From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Igor Mammedov <imammedo@redhat.com>
Cc:     kvm@vger.kernel.org, Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN
In-Reply-To: <50136685-706e-fc6a-0a77-97e584e74f93@redhat.com>
References: <20211122175818.608220-1-vkuznets@redhat.com>
 <20211122175818.608220-3-vkuznets@redhat.com>
 <16368a89-99ea-e52c-47b6-bd006933ec1f@redhat.com>
 <20211227183253.45a03ca2@redhat.com>
 <61325b2b-dc93-5db2-2d0a-dd0900d947f2@redhat.com>
 <87mtkdqm7m.fsf@redhat.com> <20220103104057.4dcf7948@redhat.com>
 <875yr1q8oa.fsf@redhat.com>
 <ceb63787-b057-13db-4624-b430c51625f1@redhat.com>
 <87o84qpk7d.fsf@redhat.com> <877dbbq5om.fsf@redhat.com>
 <5505d731-cf87-9662-33f3-08844d92877c@redhat.com>
 <20220111090022.1125ffb5@redhat.com> <87fsptnjic.fsf@redhat.com>
 <50136685-706e-fc6a-0a77-97e584e74f93@redhat.com>
Date:   Thu, 13 Jan 2022 10:27:30 +0100
Message-ID: <87bl0gnfy5.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Paolo Bonzini <pbonzini@redhat.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.

-- 
Vitaly