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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Date:   Thu, 19 May 2022 23:44:25 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.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

On Thu, May 19, 2022, Vitaly Kuznetsov wrote:
> 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.

Definitely not worth your time.  On VMX, CURRENT isn't a superset of GUEST when
EPT is enabled.  And on SVM, the flush doesn't actually occur until VM-Enter, i.e.
the redundant flush is just an extra write to svm->vmcb->control.tlb_ctl (or an
extra decrement of asid_generation).