linux-kernel - Re: [PATCH v3 1/3] KVM: fix rcu usage warning in kvm_arch_vcpu_ioctl_set

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Message-ID: <4D417006.4030201@redhat.com>
Date:	Thu, 27 Jan 2011 15:15:50 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] KVM: fix rcu usage warning in kvm_arch_vcpu_ioctl_set_sregs()

On 01/20/2011 05:27 PM, Marcelo Tosatti wrote:
> >  Before patch:
> >
> >  real    5m6.493s
> >  user    3m57.847s
> >  sys     9m7.115s
> >
> >  real    5m1.750s
> >  user    4m0.109s
> >  sys     9m10.192s
> >
> >
> >  After patch:
> >  real    5m0.140s
> >  user    3m57.956s
> >  sys     8m58.339s
> >
> >  real    4m56.314s
> >  user    4m0.303s
> >  sys     8m55.774s
>
> Nice. One disadvantageous side effect for the kvm_vcpu_kick path
> is that it can race with make_all_cpus_request, which is possibly
> doing unrelated, slower work (IPI'ing other vcpus, waiting for
> response).

I think we're fine here.  The kvm_vcpu_kick() check is

  	me = get_cpu();
  	if (cpu != me&&  (unsigned)cpu<  nr_cpu_ids&&  cpu_online(cpu))
-		if (atomic_xchg(&vcpu->guest_mode, 0))
+		if (kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE)
  			smp_send_reschedule(cpu);
  	put_cpu();


even if it did race, ->mode becomes EXITING_GUEST_MODE and we still 
avoid the IPI.  Note that make_all_cpus_request() cleverly checks for != 
OUTSIDE_GUEST_MODE, so if it loses the race with kvm_vcpu_kick(), it 
still sends the IPI to be sure the vcpu loop sees vcpu->requests in time.

> Looks ok, but lets wait for more careful reviews before applying.

Patches applied.

-- 
error compiling committee.c: too many arguments to function

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