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

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Date:   Tue, 12 Jul 2022 14:51:44 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
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
Subject: Re: [PATCH v3 06/25] KVM: x86: hyper-v: Cache
 HYPERV_CPUID_NESTED_FEATURES CPUID leaf

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?

That can be done later of course, so

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky