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 15:37:01 +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

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> 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:
>>
...
>>
>> 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.

I tried this with QEMU and nothing blew up, during CPU hotplug entries
come in the same order as the original. v3 which I've just sent
implements this suggestion.

-- 
Vitaly