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From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Avi Kivity <avi@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	kvm@vger.kernel.org, Gleb Natapov <gleb@redhat.com>,
	Peter Zijlstra <peter@programming.kicks-ass.net>
Subject: Re: KVM: x86: use smp_send_reschedule in kvm_vcpu_kick
Date: Mon, 09 Mar 2009 12:09:06 +0100	[thread overview]
Message-ID: <1236596946.8389.340.camel@laptop> (raw)
In-Reply-To: <20090309105855.GA14242@elte.hu>

On Mon, 2009-03-09 at 11:58 +0100, Ingo Molnar wrote:
> * Avi Kivity <avi@redhat.com> wrote:
> 
> > Marcelo Tosatti wrote:
> >> KVM uses a function call IPI to cause the exit of a guest running on a
> >> physical cpu. For virtual interrupt notification there is no need to
> >> wait on IPI receival, or to execute any function.
> >>
> >> This is exactly what the reschedule IPI does, without the overhead
> >> of function IPI. So use it instead of smp_call_function_single in
> >> kvm_vcpu_kick.
> >>
> >> Also change the "guest_mode" variable to a bit in vcpu->requests, and
> >> use that to collapse multiple IPI's that would be issued between the
> >> first one and zeroing of guest mode.
> >>
> >> This allows kvm_vcpu_kick to called from interrupt context.
> >>   
> >
> > Looks good.  The only worry I have is that we depend on 
> > smp_reschedule_interrupt() being a no-op.  I guess that's a 
> > reasonable assumption though.
> 
> It's a reasonable current assumption - but it might change in 
> the future - so please also put it into the changelog that KVM 
> will revert it or fix it differently if the scheduler grows some 
> functionality there.

Alternatively, do something like the below, then anybody poking at that
code will know to prod the KVM folks when they change anything.

[ I suspect other kvm arches will need to a similar comment ]

---
 arch/x86/kernel/smp.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 13f33ea..3b2e55e 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -172,6 +172,9 @@ void smp_reschedule_interrupt(struct pt_regs *regs)
 {
 	ack_APIC_irq();
 	inc_irq_stat(irq_resched_count);
+	/*
+	 * KVM uses this interrupt to force a cpu out of guest mode
+	 */
 }
 
 void smp_call_function_interrupt(struct pt_regs *regs)



  reply	other threads:[~2009-03-09 11:09 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-03-03  0:14 Marcelo Tosatti
2009-03-03  7:53 ` Zhang, Xiantao
2009-03-03 13:25   ` Marcelo Tosatti
2009-03-03 13:25     ` Gleb Natapov
2009-03-03 13:56       ` Marcelo Tosatti
2009-03-09 10:17 ` KVM: " Avi Kivity
2009-03-09 10:58   ` Ingo Molnar
2009-03-09 11:09     ` Peter Zijlstra [this message]
2009-03-09 23:23       ` Marcelo Tosatti
2009-03-10  8:01         ` Avi Kivity
2009-03-12  2:31           ` Zhang, Xiantao
2009-03-12  7:06             ` Avi Kivity
2009-03-12 11:04             ` Gleb Natapov
2009-03-13  2:15               ` Zhang, Xiantao
2009-03-19  7:38                 ` Gleb Natapov
2009-03-19  7:56                   ` Zhang, Xiantao
2009-03-19  8:11                     ` Gleb Natapov

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