linux-kernel - Re: [RFC PATCH v3 18/21] KVM: x86: Avoid gratuitous global clock reload in kvm_arch_vcpu

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Message-ID: <3776d48b-652e-409a-b991-d96d6a9e777e@xen.org>
Date: Fri, 24 May 2024 15:16:20 +0100
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, jalliste@...zon.co.uk, sveith@...zon.de,
 zide.chen@...el.com, Dongli Zhang <dongli.zhang@...cle.com>,
 Chenyi Qiang <chenyi.qiang@...el.com>
Subject: Re: [RFC PATCH v3 18/21] KVM: x86: Avoid gratuitous global clock
 reload in kvm_arch_vcpu_load()

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Commit d98d07ca7e034 ("KVM: x86: update pvclock area conditionally, on
> cpu migration") turned an unconditional KVM_REQ_CLOCK_UPDATE into a
> conditional one, if either the master clock isn't enabled *or* the vCPU
> was not previously scheduled (vcpu->cpu == -1). The commit message doesn't
> explain the latter condition, which is specifically for the master clock
> case.
> 
> Commit 0061d53daf26f ("KVM: x86: limit difference between kvmclock
> updates") later turned that into a KVM_REQ_GLOBAL_CLOCK_UPDATE to avoid
> skew between vCPUs.
> 
> In master clock mode there is no need for any of that, regardless of
> whether/where this vCPU was previously scheduled.
> 
> Do it only if (!kvm->arch.use_master_clock).
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   arch/x86/kvm/x86.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 

Reviewed-by: Paul Durrant <paul@....org>