linux-kernel - Re: [PATCH v3 17/34] KVM: x86: hyper-v: Introduce fast kvm_hv_l2_tlb_flush

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Date:   Thu, 19 May 2022 15:25:45 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 17/34] KVM: x86: hyper-v: Introduce fast
 kvm_hv_l2_tlb_flush_exposed() check

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

> On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
>> Introduce a helper to quickly check if KVM needs to handle VMCALL/VMMCALL
>> from L2 in L0 to process L2 TLB flush requests.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/include/asm/kvm_host.h | 1 +
>>  arch/x86/kvm/hyperv.c           | 6 ++++++
>>  arch/x86/kvm/hyperv.h           | 7 +++++++
>>  3 files changed, 14 insertions(+)
>> 
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index ce62fde5f4ff..168600490bd1 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -616,6 +616,7 @@ 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_features_eax; /* HYPERV_CPUID_NESTED_FEATURES.EAX */
>>  	} cpuid_cache;
>>  
>>  	struct kvm_vcpu_hv_tlb_flush_ring tlb_flush_ring[HV_NR_TLB_FLUSH_RINGS];
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index 79aabe0c33ec..68a0df4e3f66 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -2281,6 +2281,12 @@ 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_features_eax = entry->eax;
>> +	else
>> +		hv_vcpu->cpuid_cache.nested_features_eax = 0;
>>  }
>>  
>>  int kvm_hv_set_enforce_cpuid(struct kvm_vcpu *vcpu, bool enforce)
>> diff --git a/arch/x86/kvm/hyperv.h b/arch/x86/kvm/hyperv.h
>> index f593c9fd1dee..d8cb6d70dbc8 100644
>> --- a/arch/x86/kvm/hyperv.h
>> +++ b/arch/x86/kvm/hyperv.h
>> @@ -168,6 +168,13 @@ static inline void kvm_hv_vcpu_empty_flush_tlb(struct kvm_vcpu *vcpu)
>>  	tlb_flush_ring->read_idx = tlb_flush_ring->write_idx;
>>  }
>>  
>> +static inline bool kvm_hv_l2_tlb_flush_exposed(struct kvm_vcpu *vcpu)
>> +{
>> +	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
>> +
>> +	return hv_vcpu && (hv_vcpu->cpuid_cache.nested_features_eax & HV_X64_NESTED_DIRECT_FLUSH);
>> +}
>
> Tiny nipick (feel free to ignore): maybe use 'supported' instead of 'exposed',
> as we don't use this term in KVM often.
>

Indeed we don't. Basically, this is guest_cpuid_has() but for a Hyper-V
bit. I don't quite like 'supported' because we don't actually check
whether KVM or even L1 guest 'support' this feature or not, we check
whether the feature was 'exposed' to L1 so it can actually use it. I'm
going to rename this to

 guest_hv_cpuid_has_l2_tlb_flush()

then.

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

Thanks!

-- 
Vitaly