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Date:   Mon, 25 Oct 2021 16:26:08 +0200
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Subject: Re: [PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating
 halt-polling stop/deadline
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To:     Sean Christopherson <seanjc@google.com>,
        Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>
Cc:     James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        David Matlack <dmatlack@google.com>,
        Oliver Upton <oupton@google.com>,
        Jing Zhang <jingzhangos@google.com>
References: <20211009021236.4122790-1-seanjc@google.com>
 <20211009021236.4122790-17-seanjc@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211009021236.4122790-17-seanjc@google.com>
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On 09/10/21 04:12, Sean Christopherson wrote:
> Calculate the halt-polling "stop" time using "cur" instead of redoing
> ktime_get().  In the happy case where hardware correctly predicts
> do_halt_poll, "cur" is only a few cycles old.  And if the branch is
> mispredicted, arguably that extra latency should count toward the
> halt-polling time.
> 
> In all likelihood, the numbers involved are in the noise and either
> approach is perfectly ok.

Using "start" makes the change even more obvious, so:

     Calculate the halt-polling "stop" time using "start" instead of redoing
     ktime_get().  In practice, the numbers involved are in the noise (e.g.,
     in the happy case where hardware correctly predicts do_halt_poll and
     there are no interrupts, "start" is probably only a few cycles old)
     and either approach is perfectly ok.  But it's more precise to count
     any extra latency toward the halt-polling time.

Paolo