linux-kernel - Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest

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Date:	Thu, 05 Feb 2015 14:02:35 -0500
From:	Rik van Riel <riel@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Christian Borntraeger <borntraeger@...ibm.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	mtosatti@...hat.com, mingo@...nel.org, ak@...ux.intel.com,
	oleg@...hat.com, masami.hiramatsu.pt@...achi.com,
	fweisbec@...il.com, lcapitulino@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running
 KVM guest

On 02/05/2015 01:56 PM, Paul E. McKenney wrote:

> The real danger is doing neither.
> 
> On tick_nohz_full_cpu() CPUs, the exit-to-userspace code should invoke
> rcu_user_enter(), which sets some per-CPU state telling RCU to ignore
> that CPU, since it cannot possibly do host RCU read-side critical sections
> while running a guest.
> 
> In contrast, a non-tick_nohz_full_cpu() CPU doesn't let RCU
> know that it is executing in a guest or in userspace.  So the
> rcu_virt_note_context_switch() does the notification in that case.

Looking at context_tracking.h, I see the
function context_tracking_cpu_is_enabled().

That looks like it should do the right thing
in this case.
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