Re: kvm: RCU warning in async pf

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.
>