On 15.08.2012, at 03:17, Scott Wood wrote:
> On 08/14/2012 07:26 PM, Alexander Graf wrote:
>>
>> On 15.08.2012, at 02:17, Scott Wood wrote:
>>
>>> On 08/14/2012 06:04 PM, Alexander Graf wrote:
>>>> Generic KVM code might want to know whether we are inside guest context
>>>> or outside. It also wants to be able to push us out of guest context.
>>>>
>>>> Add support to the BookE code for the generic vcpu->mode field that describes
>>>> the above states.
>>>>
>>>> Signed-off-by: Alexander Graf <agraf@xxxxxxx>
>>>> ---
>>>> arch/powerpc/kvm/booke.c | 11 +++++++++++
>>>> 1 files changed, 11 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
>>>> index bcf87fe..70a86c0 100644
>>>> --- a/arch/powerpc/kvm/booke.c
>>>> +++ b/arch/powerpc/kvm/booke.c
>>>> @@ -501,6 +501,15 @@ static int kvmppc_prepare_to_enter(struct kvm_vcpu *vcpu)
>>>> continue;
>>>> }
>>>>
>>>> + if (vcpu->mode == EXITING_GUEST_MODE) {
>>>> + r = 1;
>>>> + break;
>>>> + }
>>>> +
>>>> + /* Going into guest context! Yay! */
>>>> + vcpu->mode = IN_GUEST_MODE;
>>>> + smp_wmb();
>>>> +
>>>> break;
>>>> }
>>>
>>> Normally on entry to this function mode should be OUTSIDE_GUEST_MODE,
>>> right? How could it possibly be EXITING_GUEST_MODE then, since that
>>> only replaces IN_GUEST_MODE?
>>>
>>> This doesn't match what x86 does with mode on entry. Mode is supposed
>>> to be set to IN_GUEST_MODE before requests are checked.
>>>
>>> I'm not sure what the point of EXITING_GUEST_MODE is at all, compared to
>>> just waiting until after interrupts are disabled before setting
>>> IN_GUEST_MODE (which we do on ppc, but not on x86 even though it seems
>>> like a trivial change), plus the existing ordering between mode and
>>> requests.
>>
>> Well, the only real use case I could find for the mode was the remote
>> vcpu kick. If we're not outside of guest mode, we get an IPI to
>> notify us that requests are outstanding.
>
> I'm curious why this is done so differently for broadcast requests than
> for single-cpu requests.
>
>> So I only get us into OUTSIDE_GUEST_MODE when we really exit
>> __vcpu_run, thus are in user space. That doesn't reflect what x86
>> does, right, but so doesn't our whole loop concept.
>
> OK. We still need to do ordering like x86 does, because otherwise
> there's a race where we could check requests before the request bit is
> set, and still have make_all_cpus_request see OUTSIDE_GUEST_MODE and not
> send an IPI.
Could you please send a patch showing what workflow you envision? The code as is should work, just be inefficient at times, right?
Alex
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