linux-kernel - Re: [RFC PATCH v3 15/21] KVM: x86: Allow KVM master clock mode when TSCs are offset from each other

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Date: Fri, 24 May 2024 15:10:33 +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 15/21] KVM: x86: Allow KVM master clock mode when
 TSCs are offset from each other

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> There is no reason why the KVM clock cannot be in masterclock mode when
> the TSCs are not in sync, as long as they are at the same *frequency*.
> 
> Running at a different frequency would lead to a systemic skew between
> the clock(s) as observed by different vCPUs due to arithmetic precision
> in the scaling. So that should indeed force the clock to be based on the
> host's CLOCK_MONOTONIC_RAW instead of being in masterclock mode where it
> is defined by the (or 'a') guest TSC.
> 
> But when the vCPUs merely have a different TSC *offset*, that's not a
> problem. The offset is applied to that vCPU's kvmclock->tsc_timestamp
> field, and it all comes out in the wash.
> 
> So, remove ka->nr_vcpus_matched_tsc and replace it with a new field
> ka->all_vcpus_matched_tsc which is not only changed to a boolean, but
> also now tracks that the *frequency* matches, not the precise offset.
> 
> Using a *count* was always racy because a new vCPU could be being
> created *while* kvm_track_tsc_matching() was running and comparing with
> kvm->online_vcpus. That variable is only atomic with respect to itself.
> In particular, kvm_arch_vcpu_create() runs before kvm->online_vcpus is
> incremented for the new vCPU, and kvm_arch_vcpu_postcreate() runs later.
> 
> Repurpose kvm_track_tsc_matching() to be called from kvm_set_tsc_khz(),
> and kill the cur_tsc_generation/last_tsc_generation fields which tracked
> the precise TSC matching.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   arch/x86/include/asm/kvm_host.h |   6 +-
>   arch/x86/kvm/x86.c              | 130 +++++++++++++++++---------------
>   2 files changed, 71 insertions(+), 65 deletions(-)
> 

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