From: David Woodhouse <dwmw2@infradead.org>
To: kvm <kvm@vger.kernel.org>
Cc: Paul Durrant <paul@xen.org>,
Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
linux-kernel@vger.kernel.org, "Griffoul,
Fred" <fgriffo@amazon.com>
Subject: Re: [PATCH] KVM: x86: Use fast path for Xen timer delivery
Date: Wed, 27 Sep 2023 08:42:40 +0200 [thread overview]
Message-ID: <74df3c302d26300f9114638d3ed6c2f565974bc6.camel@infradead.org> (raw)
In-Reply-To: <445abd6377cf1621a2d306226cab427820583967.camel@infradead.org>
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On Fri, 2023-09-22 at 10:20 +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.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).
>
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Hm, scratch that. It brings back the race condition Paolo talks about in
https://lore.kernel.org/kvm/0709ac62f664c0f3123fcdeabed3b79038cef3b6.camel@infradead.org/T/
Well, not *quite*... there's no race with clearing timer_expires
because I forgot to clear it at all :)
I think I'll be able to fix it up with an hrtimer_cancel() in the rare
ioctl attr_get path to avoid the race.
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prev parent reply other threads:[~2023-09-27 6:42 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-22 9:20 David Woodhouse
2023-09-22 9:22 ` Paul Durrant
2023-09-22 9:29 ` David Woodhouse
2023-09-22 9:41 ` Paul Durrant
2023-09-27 6:42 ` David Woodhouse [this message]
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