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.
> However, since we might do the vcpu_block in this loop, we will never
> really get into OUTSIDE_GUEST_MODE, right?
Not when the guest is just idling, only when it's exited to QEMU.
> Hrm. So what would you
> suggest? Do all the handle_exit in OUTSIDE_GUEST_MODE scope and then
> reenter IN_GUEST_MODE in prepare_to_enter?
I suggest leaving this optimization until a need is felt.
> We still wouldn't need an EXITING_GUEST_MODE though.
Right.
-Scott
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