From mboxrd@z Thu Jan  1 00:00:00 1970
From: Maxim Levitsky <mlevitsk@redhat.com>
Date: Wed, 27 Oct 2021 14:35:45 +0000
Subject: Re: [PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline
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To: Paolo Bonzini <pbonzini@redhat.com>, 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>

On Mon, 2021-10-25 at 16:26 +0200, Paolo Bonzini wrote:
> 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
> 
Agreed.

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