linux-kernel - Re: [PATCH v2 04/11] KVM: x86: Process "guest stopped request" once per guest time update

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Message-ID: <acc56a9b-2313-43f8-8acf-9ec39a8538e8@xen.org>
Date: Tue, 4 Feb 2025 09:19:58 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>, David Woodhouse <dwmw2@...radead.org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzbot+352e553a86e0d75f5120@...kaller.appspotmail.com,
 Paul Durrant <pdurrant@...zon.com>, David Woodhouse <dwmw@...zon.co.uk>,
 Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v2 04/11] KVM: x86: Process "guest stopped request" once
 per guest time update

On 01/02/2025 01:38, Sean Christopherson wrote:
> Handle "guest stopped" requests once per guest time update in preparation
> of restoring KVM's historical behavior of setting PVCLOCK_GUEST_STOPPED
> for kvmclock and only kvmclock.  For now, simply move the code to minimize
> the probability of an unintentional change in functionally.
> 
> Note, in practice, all clocks are guaranteed to see the request (or not)
> even though each PV clock processes the request individual, as KVM holds
> vcpu->mutex (blocks KVM_KVMCLOCK_CTRL) and it should be impossible for
> KVM's suspend notifier to run while KVM is handling requests.  And because
> the helper updates the reference flags, all subsequent PV clock updates
> will pick up PVCLOCK_GUEST_STOPPED.
> 
> Note #2, once PVCLOCK_GUEST_STOPPED is restricted to kvmclock, the
> horrific #ifdef will go away.
> 

:-)

> Cc: Paul Durrant <pdurrant@...zon.com>
> Cc: David Woodhouse <dwmw@...zon.co.uk>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/x86.c | 17 ++++++++++++-----
>   1 file changed, 12 insertions(+), 5 deletions(-)
> 

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