linux-kernel - Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

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Date:   Mon, 2 Oct 2023 11:35:38 +0100
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>, kvm <kvm@...r.kernel.org>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

On 30/09/2023 14:58, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Most of the time there's no need to kick the vCPU and deliver the timer
> event through kvm_xen_inject_timer_irqs(). Use kvm_xen_set_evtchn_fast()
> directly from the timer callback, and only fall back to the slow path
> when it's necessary to do so.
> 
> This gives a significant improvement in timer latency testing (using
> nanosleep() for various periods and then measuring the actual time
> elapsed).
> 
> However, there was a reason¹ the fast path was dropped when this support
> was first added. The current code holds vcpu->mutex for all operations
> on the kvm->arch.timer_expires field, and the fast path introduces a
> potential race condition. Avoid that race by ensuring the hrtimer is
> (temporarily) cancelled before making changes in kvm_xen_start_timer(),
> and also when reading the values out for KVM_XEN_VCPU_ATTR_TYPE_TIMER.
> 
> ¹ https://lore.kernel.org/kvm/846caa99-2e42-4443-1070-84e49d2f11d2@redhat.com/
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   • v2: Remember, and deal with, those races.
> 
>   • v3: Drop the assertions for vcpu being loaded; those can be done
>         separately if at all.
> 
>         Reorder the code in xen_timer_callback() to make it clearer
>         that kvm->arch.xen.timer_expires is being cleared in the case
>         where the event channel delivery is *complete*, as opposed to
>         the -EWOULDBLOCK deferred path.
> 
>         Drop the 'pending' variable in kvm_xen_vcpu_get_attr() and
>         restart the hrtimer if (kvm->arch.xen.timer_expires), which
>         ought to be exactly the same thing (that's the *point* in
>         cancelling the timer, to make it truthful as we return its
>         value to userspace).
> 
>         Improve comments.
> 
>   arch/x86/kvm/xen.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 49 insertions(+)
> 

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