linux-kernel - Re: [RFC PATCH v3 17/21] KVM: x86: Avoid global clock update on setting KVM clock MSR

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Message-ID: <Zr7Vh74T5yIVQyqi@google.com>
Date: Thu, 15 Aug 2024 21:28:55 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Jonathan Corbet <corbet@....net>, 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>, Paul Durrant <paul@....org>, 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 17/21] KVM: x86: Avoid global clock update on
 setting KVM clock MSR

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Commit 0061d53daf26 ("KVM: x86: limit difference between kvmclock updates")
> introduced a KVM_REQ_GLOBAL_CLOCK_UPDATE when one vCPU set up its clock.
> 
> This was a workaround for the ever-drifting clocks which were based on the
> host's CLOCK_MONOTONIC and thus subject to NTP skew. On booting or resuming
> a guest, it just leads to running kvm_guest_time_update() twice for each
> vCPU for now good reason.

s/now/no

> Just use KVM_REQ_CLOCK_UPDATE on the vCPU itself, and only in the case
> where the KVM clock is being set up, not turned off.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  arch/x86/kvm/x86.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 437412b36cae..32a873d5ed00 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2361,13 +2361,13 @@ static void kvm_write_system_time(struct kvm_vcpu *vcpu, gpa_t system_time,
>  	}
>  
>  	vcpu->arch.time = system_time;
> -	kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
>  
>  	/* we verify if the enable bit is set... */
> -	if (system_time & 1)
> +	if (system_time & 1) {
>  		kvm_gpc_activate(&vcpu->arch.pv_time, system_time & ~1ULL,
>  				 sizeof(struct pvclock_vcpu_time_info));
> -	else
> +		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> +	} else

Needs curly braces now that the if-statement has 'em.

>  		kvm_gpc_deactivate(&vcpu->arch.pv_time);
>  
>  	return;
> -- 
> 2.44.0
>