linux-kernel - Re: [PATCH v2] KVM: x86: Use fast path for Xen timer delivery

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Date:   Fri, 29 Sep 2023 14:26:05 +0100
From:   "Durrant, Paul" <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>, kvm <kvm@...r.kernel.org>
Cc:     Paul Durrant <paul@....org>,
        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 v2] KVM: x86: Use fast path for Xen timer delivery

On 29/09/2023 12:36, 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 potential
> race conditions. So... ensure the hrtimer is *cancelled* before making
> changes in kvm_xen_start_timer(), and also when reading the values out
> for KVM_XEN_VCPU_ATTR_TYPE_TIMER.
> 
> Add some sanity checks to ensure the truth of the claim that all the
> other code paths are run with the vcpu loaded. And use hrtimer_cancel()
> directly from kvm_xen_destroy_vcpu() to avoid a false positive from the
> check in kvm_xen_stop_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.
> 
>   arch/x86/kvm/xen.c | 64 +++++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 58 insertions(+), 6 deletions(-)
> 

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