linux-kernel - Re: [PATCH v3 20/34] KVM: x86: KVM_REQ_TLB_FLUSH_CURRENT is a superset of KVM_REQ_HV_TLB

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Message-ID: <87mtfdubro.fsf@redhat.com>
Date:   Thu, 19 May 2022 11:12:11 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...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,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 20/34] KVM: x86: KVM_REQ_TLB_FLUSH_CURRENT is a
 superset of KVM_REQ_HV_TLB_FLUSH too

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

> On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
>> KVM_REQ_TLB_FLUSH_CURRENT is an even stronger operation than
>> KVM_REQ_TLB_FLUSH_GUEST so KVM_REQ_HV_TLB_FLUSH needs not to be
>> processed after it.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/x86.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index e5aec386d299..d3839e648ab3 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3357,8 +3357,11 @@ static inline void kvm_vcpu_flush_tlb_current(struct kvm_vcpu *vcpu)
>>   */
>>  void kvm_service_local_tlb_flush_requests(struct kvm_vcpu *vcpu)
>>  {
>> -	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
>> +	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu)) {
>>  		kvm_vcpu_flush_tlb_current(vcpu);
>> +		if (kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu))
>> +			kvm_hv_vcpu_empty_flush_tlb(vcpu);
>> +	}
>>  
>>  	if (kvm_check_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu)) {
>>  		kvm_vcpu_flush_tlb_guest(vcpu);
>
>
> I think that this patch should be moved near patch 1 and/or even squished with it.
>

Sure, will merge.

This, however, made me think there's room for optimization here. In some
cases, when both KVM_REQ_TLB_FLUSH_CURRENT and KVM_REQ_TLB_FLUSH_GUEST
were requested, there's no need to flush twice, e.g. on SVM
.flush_tlb_current == .flush_tlb_guest. I'll probably not go into this
territory with this series as it's already fairly big, just something
for the future.

-- 
Vitaly