From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: kvm: RCU warning in async pf
Date: Wed, 4 Apr 2012 07:04:16 -0700
Message-ID: <20120404140416.GF2302@linux.vnet.ibm.com>
References: <1333414472.11455.7.camel@lappy>
 <20120403105226.GF14939@redhat.com>
 <20120404123033.GD11918@redhat.com>
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Cc: Sasha Levin <levinsasha928@gmail.com>, Avi Kivity <avi@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@elte.hu>, kvm <kvm@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Dave Jones <davej@redhat.com>
To: Gleb Natapov <gleb@redhat.com>
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On Wed, Apr 04, 2012 at 03:30:33PM +0300, Gleb Natapov wrote:
> 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.

This is invoked from an exception or interrupt handler, not from
process-level code?  If so:

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Gleb Natapov <gleb@redhat.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.
>