linux-kernel - Re: [PATCH 2/2] KVM: make guest mode entry to be rcu quiescent state

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Message-ID: <20110503154240.GA6898@amt.cnet>
Date:	Tue, 3 May 2011 12:42:41 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	avi@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: make guest mode entry to be rcu quiescent state

On Tue, May 03, 2011 at 04:39:01PM +0300, Gleb Natapov wrote:
> > > > On Thu, Apr 28, 2011 at 12:52:03PM +0300, Gleb Natapov wrote:
> > > > > KVM does not hold any references to rcu protected data when it switches
> > > > > CPU into a guest mode. In fact switching to a guest mode is very similar
> > > > > to exiting to userspase from rcu point of view. In addition CPU may stay
> > > > > in a guest mode for quite a long time (up to one time slice). Lets treat
> > > > > guest mode as quiescent state, just like we do with user-mode execution.
> > > > > 
> > > > > Signed-off-by: Gleb Natapov <gleb@...hat.com>
> > > > > ---
> > > > >  include/linux/kvm_host.h |    1 +
> > > > >  1 files changed, 1 insertions(+), 0 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > > > index 0bc3d37..a347bce 100644
> > > > > --- a/include/linux/kvm_host.h
> > > > > +++ b/include/linux/kvm_host.h
> > > > > @@ -593,6 +593,7 @@ static inline void kvm_guest_enter(void)
> > > > >  {
> > > > >  	account_system_vtime(current);
> > > > >  	current->flags |= PF_VCPU;
> > > > > +	rcu_note_context_switch(smp_processor_id());
> > > > >  }
> > > > >  
> > > > >  static inline void kvm_guest_exit(void)
> > > > > -- 
> > > > > 1.7.2.3
> > > > 
> > > > Please have it in x86's vcpu_enter_guest, then its more explicit
> > > > (uncertain about the context of kvm_guest_enter call in other arches).
> > > > 
> > > I checked all of them and kvm_guest_enter() is always called with local
> > > irq disabled. Paul confirmed that rcu_note_context_switch() can be
> > > called in such context.
> > 
> > OK then. Perhaps have an assert to verify interrupts are disabled?
> Yes. Can add BUG_ON(preemptible()).

Also please add a comment to explain whats going on. The commit message
above seems appropriate.

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