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From: Paul Durrant <xadimgnik@gmail.com>
Message-ID: <c36f5401-05e9-4c13-a24e-31ebab8ae4f7@xen.org>
Date: Fri, 24 May 2024 15:10:33 +0100
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Subject: Re: [RFC PATCH v3 15/21] KVM: x86: Allow KVM master clock mode when
 TSCs are offset from each other
To: David Woodhouse <dwmw2@infradead.org>, kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
 Sean Christopherson <seanjc@google.com>, Thomas Gleixner
 <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
 Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>,
 Vincent Guittot <vincent.guittot@linaro.org>,
 Dietmar Eggemann <dietmar.eggemann@arm.com>,
 Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>,
 Mel Gorman <mgorman@suse.de>, Daniel Bristot de Oliveira
 <bristot@redhat.com>, Valentin Schneider <vschneid@redhat.com>,
 Shuah Khan <shuah@kernel.org>, linux-doc@vger.kernel.org,
 linux-kernel@vger.kernel.org, jalliste@amazon.co.uk, sveith@amazon.de,
 zide.chen@intel.com, Dongli Zhang <dongli.zhang@oracle.com>,
 Chenyi Qiang <chenyi.qiang@intel.com>
References: <20240522001817.619072-1-dwmw2@infradead.org>
 <20240522001817.619072-16-dwmw2@infradead.org>
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On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.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@amazon.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@xen.org>