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    SubjectRe: 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@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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