linux-kernel - Re: RCU+KVM: making CPU guest mode a quiescent state.

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Message-ID: <20110427223209.GW2135@linux.vnet.ibm.com>
Date:	Wed, 27 Apr 2011 15:32:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	avi@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: RCU+KVM: making CPU guest mode a quiescent state.

On Wed, Apr 27, 2011 at 03:41:41PM +0300, Gleb Natapov wrote:
> On Tue, Apr 26, 2011 at 08:55:28AM -0700, Paul E. McKenney wrote:
> > On Tue, Apr 26, 2011 at 03:38:24PM +0300, Gleb Natapov wrote:
> > > Hello Paul,
> > > 
> > > I have a question about RCU + KVM. 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). It looks like it will be beneficial to treat guest
> > > mode as quiescent state, just like user-mode execution. How can this be
> > > done? I was trying to find how RCU knows about cpu entering user-mode,
> > > but it seems that it does this by checking CPU mode in a timer interrupt
> > > (update_process_times()->rcu_check_callbacks()). This will not work for
> > > guest mode detection since timer interrupt will kick CPU out of a guest
> > > mode and timer interrupt will always see CPU in kernel mode. Do we have
> > > a simple function to call to notify RCU that CPU passed quiescent state
> > > which we can call just before entering guest?
> > 
> > Hello, Gleb,
> > 
> > You could call rcu_note_context_switch(), passing it the current
> > CPU.  Please note that preemption -must- be disabled when calling
> > this.  You could call this just after exiting the guest as well
> > as just before entering guest.
> > 
> x86 disable preemption and local interrupts before switching to guest
> mode. Some platforms only disable local interrupts. I assume disabling
> local irqs is as good as disabling preemption for rcu_note_context_switch()
> calling purpose, is this correct?

Yep, disabing hardirqs will suffice.

							Thanx, Paul
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