On 08/15/2012 04:29 AM, Alexander Graf wrote:
>
> 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?
I'll try to do this tomorrow.
> The code as is should work, just be inefficient at times, right?
No, we could fail to send the IPI -- couldn't that result in the guest
using a stale TLB entry?
-Scott
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