linux-kernel - Re: kvm: RCU warning in async pf

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Date:	Wed, 4 Apr 2012 15:30:33 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	kvm <kvm@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: kvm: RCU warning in async pf

On Tue, Apr 03, 2012 at 01:52:26PM +0300, Gleb Natapov wrote:
> On Mon, Apr 02, 2012 at 08:54:32PM -0400, Sasha Levin wrote:
> > Hi all,
> > 
> > I got the spew at the bottom of the mail in a KVM guest using the KVM tools and running trinity.
> > 
> > I'm not quite sure how default_idle managed to trigger a pagefault, so that part looks odd to me.
> > 
> This is not regular page fault. This is async page fault that tells the
> guest that a page, previously swapped out by hypervisor, is now swapped
> back in and it can happen while vcpu is idle. The code does not leave
> idle state properly though. We probably need to call rcu_irq_enter()
> there. Will look into it.
> 

The patch below solves it for me:

"Page ready" async PF can kick vcpu out of idle state much like IRQ.
We need to tell RCU about this.

Signed-off-by: Gleb Natapov <gleb@...hat.com>
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index f0c6fd6..380079f 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -38,6 +38,7 @@
 #include <asm/traps.h>
 #include <asm/desc.h>
 #include <asm/tlbflush.h>
+#include <asm/idle.h>
 
 static int kvmapf = 1;
 
@@ -253,7 +254,10 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
 		kvm_async_pf_task_wait((u32)read_cr2());
 		break;
 	case KVM_PV_REASON_PAGE_READY:
+		rcu_irq_enter();
+		exit_idle();
 		kvm_async_pf_task_wake((u32)read_cr2());
+		rcu_irq_exit();
 		break;
 	}
 }
--
			Gleb.
--
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