On 08.01.2012, at 16:22, Avi Kivity wrote:
> On 01/08/2012 05:11 PM, Alexander Graf wrote:
>> On 08.01.2012, at 14:18, Avi Kivity wrote:
>>
>>> On 01/04/2012 03:10 AM, Alexander Graf wrote:
>>>> From: Scott Wood <scottwood@xxxxxxxxxxxxx>
>>>>
>>>> Currently we check prior to returning from a lightweight exit,
>>>> but not prior to initial entry.
>>>>
>>>> book3s already does a similar test.
>>>>
>>>> Signed-off-by: Scott Wood <scottwood@xxxxxxxxxxxxx>
>>>> Signed-off-by: Alexander Graf <agraf@xxxxxxx>
>>>> ---
>>>> arch/powerpc/kvm/booke.c | 10 +++++++++-
>>>> 1 files changed, 9 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
>>>> index b642200..9c78589 100644
>>>> --- a/arch/powerpc/kvm/booke.c
>>>> +++ b/arch/powerpc/kvm/booke.c
>>>> @@ -322,11 +322,19 @@ int kvmppc_vcpu_run(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
>>>> }
>>>>
>>>> local_irq_disable();
>>>> +
>>>> + if (signal_pending(current)) {
>>>> + kvm_run->exit_reason = KVM_EXIT_INTR;
>>>> + ret = -EINTR;
>>>> + goto out;
>>>> + }
>>>> +
>>>> kvm_guest_enter();
>>>> ret = __kvmppc_vcpu_run(kvm_run, vcpu);
>>>> kvm_guest_exit();
>>>> - local_irq_enable();
>>>>
>>>>
>>>
>>> In general a single check prior to entry is sufficient (well, in
>>> addition to the one in kvm_vcpu_block()).
>>
>> Yes, and IIUC this is the single check prior to entry. On lightweight exit, we don't return from __kvmppc_vcpu_run, but only call kvmppc_handle_exit() and if that exits that we return to the guest, we stay inside of __kvmppc_vcpu_run and don't return from here.
>
> It means you check twice per heavyweight exit, no? Once here, and once
> when kvmppc_handle_exit() returns. If, instead, you move the check to
> just before the lightweight entry, you check just once per entry, like x86.
You mean we check twice in case a heavyweight exit occurs right after another heavyweight exit? We need to check whether a signal is pending to determine heavyweight exits, so we definitely have to check at the end of the exit handlers:
- check signal (none pending)
- enter guest
- exit guest because an external interrupt occurs (lightweight)
- check signal (pending now because we got preempted)
-> make exit heavyweight
If however there already is a signal pending before we enter the guest and the guest is running an endless loop and the host doesn't have any timers configured because it's just waiting for the signal to be handled, we would be in a dead loop. So we have to check before entry either way.
Alex
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