linux-kernel - Re: [RFC PATCH v3 20/21] KVM: x86/xen: Prevent runstate times from becoming negative

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Date: Fri, 24 May 2024 15:21:47 +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 20/21] KVM: x86/xen: Prevent runstate times from
 becoming negative

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> When kvm_xen_update_runstate() is invoked to set a vCPU's runstate, the
> time spent in the previous runstate is accounted. This is based on the
> delta between the current KVM clock time, and the previous value stored
> in vcpu->arch.xen.runstate_entry_time.
> 
> If the KVM clock goes backwards, that delta will be negative. Or, since
> it's an unsigned 64-bit integer, very *large*. Linux guests deal with
> that particularly badly, reporting 100% steal time for ever more (well,
> for *centuries* at least, until the delta has been consumed).
> 
> So when a negative delta is detected, just refrain from updating the
> runstates until the KVM clock catches up with runstate_entry_time again.
> 
> The userspace APIs for setting the runstate times do not allow them to
> be set past the current KVM clock, but userspace can still adjust the
> KVM clock *after* setting the runstate times, which would cause this
> situation to occur.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   arch/x86/kvm/xen.c | 18 ++++++++++++++----
>   1 file changed, 14 insertions(+), 4 deletions(-)
> 

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