Re: kvm: RCU warning in async pf

[Gleb Natapov <gleb@redhat.com>]

On Wed, Apr 04, 2012 at 07:04:16AM -0700, Paul E. McKenney wrote:
> 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:
> 
>From an exception.

> 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.
> > 
> --
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--
			Gleb.