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    Subject[ PATCH] Add rcu user eqs exception hooks for async page fault
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    This patch adds user eqs exception hooks for async page fault page not
    present code path, to exit the user eqs and re-enter it as necessary.

    Async page fault is different from other exceptions that it may be
    triggered from idle process, so we still need rcu_irq_enter() and
    rcu_irq_exit() to exit cpu idle eqs when needed, to protect the code
    that needs use rcu.

    As Frederic pointed out it would be safest and simplest to protect the
    whole kvm_async_pf_task_wait(). Otherwise, "we need to check all the
    code there deeply for potential RCU uses and ensure it will never be
    extended later to use RCU.".

    However, We'd better re-enter the cpu idle eqs if we get the exception
    in cpu idle eqs, by calling rcu_irq_exit() before native_safe_halt().

    So the patch does what Frederic suggested for rcu_irq_*() API usage
    here, except that I moved the rcu_irq_*() pair originally in
    do_async_page_fault() into kvm_async_pf_task_wait().

    That's because, I think it's better to have rcu_irq_*() pairs to be in
    one function ( rcu_irq_exit() after rcu_irq_enter() ), especially here,
    kvm_async_pf_task_wait() has other callers, which might cause
    rcu_irq_exit() be called without a matching rcu_irq_enter() before it,
    which is illegal if the cpu happens to be in rcu idle state.

    Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
    ---
    arch/x86/kernel/kvm.c | 12 ++++++++++--
    1 file changed, 10 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
    index 08b973f..e99af60 100644
    --- a/arch/x86/kernel/kvm.c
    +++ b/arch/x86/kernel/kvm.c
    @@ -43,6 +43,7 @@
    #include <asm/apicdef.h>
    #include <asm/hypervisor.h>
    #include <asm/kvm_guest.h>
    +#include <asm/rcu.h>

    static int kvmapf = 1;

    @@ -121,6 +122,8 @@ void kvm_async_pf_task_wait(u32 token)
    struct kvm_task_sleep_node n, *e;
    DEFINE_WAIT(wait);

    + rcu_irq_enter();
    +
    spin_lock(&b->lock);
    e = _find_apf_task(b, token);
    if (e) {
    @@ -128,6 +131,8 @@ void kvm_async_pf_task_wait(u32 token)
    hlist_del(&e->link);
    kfree(e);
    spin_unlock(&b->lock);
    +
    + rcu_irq_exit();
    return;
    }

    @@ -152,13 +157,16 @@ void kvm_async_pf_task_wait(u32 token)
    /*
    * We cannot reschedule. So halt.
    */
    + rcu_irq_exit();
    native_safe_halt();
    + rcu_irq_enter();
    local_irq_disable();
    }
    }
    if (!n.halted)
    finish_wait(&n.wq, &wait);

    + rcu_irq_exit();
    return;
    }
    EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
    @@ -252,10 +260,10 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
    break;
    case KVM_PV_REASON_PAGE_NOT_PRESENT:
    /* page is swapped out by the host. */
    - rcu_irq_enter();
    + exception_enter(regs);
    exit_idle();
    kvm_async_pf_task_wait((u32)read_cr2());
    - rcu_irq_exit();
    + exception_exit(regs);
    break;
    case KVM_PV_REASON_PAGE_READY:
    rcu_irq_enter();
    --
    1.7.11.4


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