linux-kernel - Re: [PATCH v3 06/25] KVM: x86: hyper-v: Cache HYPERV_CPUID_NESTED

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Date:   Wed, 13 Jul 2022 17:45:23 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v3 06/25] KVM: x86: hyper-v: Cache
 HYPERV_CPUID_NESTED_FEATURES CPUID leaf

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

> On Fri, 2022-07-08 at 16:42 +0200, Vitaly Kuznetsov wrote:
>> KVM has to check guest visible HYPERV_CPUID_NESTED_FEATURES.EBX CPUID
>> leaf to know with Enlightened VMCS definition to use (original or 2022
>> update). Cache the leaf along with other Hyper-V CPUID feature leaves
>> to make the check quick.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/include/asm/kvm_host.h | 2 ++
>>  arch/x86/kvm/hyperv.c           | 9 +++++++++
>>  2 files changed, 11 insertions(+)
>> 
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index de5a149d0971..077ec9cf3169 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -616,6 +616,8 @@ struct kvm_vcpu_hv {
>>  		u32 enlightenments_eax; /* HYPERV_CPUID_ENLIGHTMENT_INFO.EAX */
>>  		u32 enlightenments_ebx; /* HYPERV_CPUID_ENLIGHTMENT_INFO.EBX */
>>  		u32 syndbg_cap_eax; /* HYPERV_CPUID_SYNDBG_PLATFORM_CAPABILITIES.EAX */
>> +		u32 nested_eax; /* HYPERV_CPUID_NESTED_FEATURES.EAX */
>> +		u32 nested_ebx; /* HYPERV_CPUID_NESTED_FEATURES.EBX */
>>  	} cpuid_cache;
>>  };
>>  
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index e08189211d9a..b666902da4d9 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -2030,6 +2030,15 @@ void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu)
>>  		hv_vcpu->cpuid_cache.syndbg_cap_eax = entry->eax;
>>  	else
>>  		hv_vcpu->cpuid_cache.syndbg_cap_eax = 0;
>> +
>> +	entry = kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_NESTED_FEATURES, 0);
>> +	if (entry) {
>> +		hv_vcpu->cpuid_cache.nested_eax = entry->eax;
>> +		hv_vcpu->cpuid_cache.nested_ebx = entry->ebx;
>> +	} else {
>> +		hv_vcpu->cpuid_cache.nested_eax = 0;
>> +		hv_vcpu->cpuid_cache.nested_ebx = 0;
>> +	}
>>  }
>>  
>>  int kvm_hv_set_enforce_cpuid(struct kvm_vcpu *vcpu, bool enforce)
>
>
> Small Nitpick:
>
> If I understand correctly, the kvm_find_cpuid_entry can fail if the userspace didn't provide the
> cpuid entry.
>
> Since the code that deals with failback is now repeated 3 times, how about some wrapper function that
> will return all zeros for a non present cpuid entry?

I've opted for wiping the whole hv_vcpu->cpuid_cache with memset(), this
way we don't even need a new helper.

-- 
Vitaly