linux-kernel - Re: [PATCH v2 2/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM

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Date:   Tue, 18 Jan 2022 09:40:53 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] KVM: x86: Partially allow KVM_SET_CPUID{,2}
 after KVM_RUN

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 1/17/22 16:05, Vitaly Kuznetsov wrote:
>>   
>> +/* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */
>> +static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>> +				 int nent)
>> +{
>> +	struct kvm_cpuid_entry2 *best;
>> +	int i;
>> +
>> +	for (i = 0; i < nent; i++) {
>> +		best = kvm_find_cpuid_entry(vcpu, e2[i].function, e2[i].index);
>> +		if (!best)
>> +			return -EINVAL;
>> +
>> +		if (e2[i].eax != best->eax || e2[i].ebx != best->ebx ||
>> +		    e2[i].ecx != best->ecx || e2[i].edx != best->edx)
>> +			return -EINVAL;
>> +	}
>> +
>> +	return 0;
>> +}
>
> What about this alternative implementation:
>
> /* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */
> static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>                                   int nent)
> {
>          struct kvm_cpuid_entry2 *orig;
>          int i;
>
>          if (nent != vcpu->arch.cpuid_nent)
>                  return -EINVAL;
>
>          for (i = 0; i < nent; i++) {
>                  orig = &vcpu->arch.cpuid_entries[i];
>                  if (e2[i].function != orig->function ||
>                      e2[i].index != orig->index ||
>                      e2[i].eax != orig->eax || e2[i].ebx != orig->ebx ||
>                      e2[i].ecx != orig->ecx || e2[i].edx != orig->edx)
>                          return -EINVAL;
>          }
>
>          return 0;
> }
>
> avoiding the repeated calls to kvm_find_cpuid_entry?
>

My version is a bit more permissive as it allows supplying CPUID entries
in any order, not necessarily matching the original. I *guess* this
doesn't matter much for the QEMU problem we're trying to workaround,
I'll have to check.

-- 
Vitaly