linux-kernel - Re: [RFC PATCH v3 10/21] KVM: x86: Fix software TSC upscaling in kvm_update_guest

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Date: Fri, 24 May 2024 14:26:05 +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 10/21] KVM: x86: Fix software TSC upscaling in
 kvm_update_guest_time()

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> There was some confusion in kvm_update_guest_time() when software needs
> to advance the guest TSC.
> 
> In master clock mode, there are two points of time which need to be taken
> into account. First there is the master clock reference point, stored in
> kvm->arch.master_kernel_ns (and associated host TSC ->master_cycle_now).
> Secondly, there is the time *now*, at the point kvm_update_guest_time()
> is being called.
> 
> With software TSC upscaling, the guest TSC is getting further and further
> ahead of the host TSC as time elapses. So at time "now", the guest TSC
> should be further ahead of the host, than it was at master_kernel_ns.
> 
> The adjustment in kvm_update_guest_time() was not taking that into
> account, and was only advancing the guest TSC by the appropriate amount
> for master_kernel_ns, *not* the current time.
> 
> Fix it to calculate them both correctly.
> 
> Since the KVM clock reference point in master_kernel_ns might actually
> be *earlier* than the reference point used for the guest TSC
> (vcpu->last_tsc_nsec), this might lead to a negative delta. Fix the
> compute_guest_tsc() function to cope with negative numbers, which
> then means there is no need to force a master clock update when the
> guest TSC is written.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk > ---
>   arch/x86/kvm/x86.c | 73 +++++++++++++++++++++++++++++++++++-----------
>   1 file changed, 56 insertions(+), 17 deletions(-)
> 

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