linux-kernel - Re: [PATCH 02/10] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force

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Date: Fri, 19 Apr 2024 16:29:51 +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>,
 Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Oliver Upton <oliver.upton@...ux.dev>, Marcelo Tosatti
 <mtosatti@...hat.com>, jalliste@...zon.co.uk, sveith@...zon.de
Subject: Re: [PATCH 02/10] KVM: x86: Improve accuracy of KVM clock when TSC
 scaling is in force

On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> The kvm_guest_time_update() function scales the host TSC frequency to
> the guest's using kvm_scale_tsc() and the v->arch.l1_tsc_scaling_ratio
> scaling ratio previously calculated for that vCPU. Then calcuates the
> scaling factors for the KVM clock itself based on that guest TSC
> frequency.
> 
> However, it uses kHz as the unit when scaling, and then multiplies by
> 1000 only at the end.
> 
> With a host TSC frequency of 3000MHz and a guest set to 2500MHz, the
> result of kvm_scale_tsc() will actually come out at 2,499,999kHz. So
> the KVM clock advertised to the guest is based on a frequency of
> 2,499,999,000 Hz.
> 
> By using Hz as the unit from the beginning, the KVM clock would be based
> on a more accurate frequency of 2,499,999,999 Hz in this example.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> Fixes: 78db6a503796 ("KVM: x86: rewrite handling of scaled TSC for kvmclock")
> ---
>   arch/x86/include/asm/kvm_host.h |  2 +-
>   arch/x86/kvm/x86.c              | 17 +++++++++--------
>   arch/x86/kvm/xen.c              |  2 +-
>   3 files changed, 11 insertions(+), 10 deletions(-)
> 

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