On 04/04/2017 19:19, Christoffer Dall wrote:
> On Tue, Apr 04, 2017 at 06:24:36PM +0200, Paolo Bonzini wrote:
>>
>>
>> On 04/04/2017 18:04, Christoffer Dall wrote:
>>>> For pause, only the requester should do the clearing.
>>
>> This suggests that maybe this should not be a request. The request
>> would be just the need to act on a GIC command, exactly as before this patch.
>
> Maybe the semantics should be:
>
> requester: vcpu:
> ---------- -----
> make_requet(vcpu, KVM_REQ_PAUSE);
> handles the request by
> clearing it and setting
> vcpu->pause = true;
> wait until vcpu->pause == true
> make_request(vcpu, KVM_REQ_UNPAUSE);
> vcpus 'wake up' clear the
> UNPAUSE request and set
> vcpu->pause = false;
>
> The benefit would be that we get to re-use the complicated "figure out
> the VCPU mode and whether or not we should send an IPI and get the
> barriers right" stuff.
I don't think that's necessary. As long as the complicated stuff avoids
that you enter the VCPU, the next run through the loop will
find that 'vcpu->arch.power_off || vcpu->arch.pause' is true and
go to sleep.
>> What I don't understand is:
>>
>>>> With this patch, while the vcpu will still initially enter
>>>> the guest, it will exit immediately due to the IPI sent by the vcpu
>>>> kick issued after making the vcpu request.
>>
>> Isn't this also true of KVM_REQ_VCPU_EXIT that was used before?
>>
>> So this:
>>
>> + vcpu->arch.power_off || kvm_request_pending(vcpu)) {
>> + WRITE_ONCE(vcpu->mode, OUTSIDE_GUEST_MODE);
>>
>> is the crux of the fix, you can keep using vcpu->arch.pause.
>
> Probably; I feel like there's a fix here which should be a separate
> patch from using a different requests instead of the KVM_REQ_VCPU_EXIT +
> the pause flag.
Yeah, and then the pause flag can stay.
>> By the way, vcpu->arch.power_off can go away from this "if" too because
>> KVM_RUN and KVM_SET_MP_STATE are mutually exclusive through the vcpu mutex.
>
> But we also allow setting the power_off flag from the in-kernel PSCI
> emulation in the context of another VCPU thread.
Right. That code does
tmp->arch.power_off = true;
kvm_vcpu_kick(tmp);
and I think what's really missing in arm.c is the "if (vcpu->mode ==
EXITING_GUEST_MODE)" check that is found in x86.c. Then pausing can
also simply use kvm_vcpu_kick.
My understanding is that KVM-ARM is using KVM_REQ_VCPU_EXIT simply to
reuse the smp_call_function_many code in kvm_make_all_cpus_request.
Once you add EXITING_GUEST_MODE, ARM can just add a new function
kvm_kick_all_cpus and use it for both pause and power_off.
Paolo
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