Re: [PATCH v3 05/10] KVM: arm/arm64: don't clear exit request from caller

Christoffer Dall
2017-05-10T09:55:18Z

On Tue, May 09, 2017 at 07:17:06PM +0200, Andrew Jones wrote:
> On Sat, May 06, 2017 at 08:12:56PM +0200, Christoffer Dall wrote:
> > On Wed, May 03, 2017 at 06:06:30PM +0200, Andrew Jones wrote:
> > > VCPU requests that the receiver should handle should only be cleared
> > > by the receiver.
> >
> > I cannot parse this sentence.
>
> I'll try again:
>
> VCPU requests should only be cleared by the receiving VCPUs. The only
> exception is when a request is set as a side-effect. In these cases
> the "requester" threads may clear the requests when it is sure the
> receiving VCPUs do not need to see them.
>

I can parse this, and I mostly understand this, except for the part
about side-effects.

> >
> > > Not only does this properly implement the protocol,
> > > but also avoids bugs where one VCPU clears another VCPU's request,
> > > before the receiving VCPU has had a chance to see it.
> >
> > Is this an actual race we have currently or just something thay may
> > happen later. Im' not sure.
>
> Since ARM is just learning to handle VCPU requests, then it's not a bug
> now. Actually, I think I should state this protocol (what I wrote above)
> in the document, and then I can just reference that here in this commit
> message as the justification for change.

That might solve the missing piece for me above, yes.

> >
> > > ARM VCPUs
> > > currently only handle one request, EXIT, and handling it is achieved
> > > by checking pause to see if the VCPU should sleep.
> >
> > This makes sense. So forget my comment on the previous patch about
> > getting rid of the pause flag.
>
> Forgotten
>
> >
> > >
> > > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > > ---
> > > arch/arm/kvm/arm.c | 10 ++++++++--
> > > 1 file changed, 8 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> > > index 9174ed13135a..7be0d9b0c63a 100644
> > > --- a/arch/arm/kvm/arm.c
> > > +++ b/arch/arm/kvm/arm.c
> > > @@ -553,7 +553,6 @@ void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu)
> > > {
> > > struct swait_queue_head *wq = kvm_arch_vcpu_wq(vcpu);
> > >
> > > - kvm_clear_request(KVM_REQ_VCPU_EXIT, vcpu);
> > > vcpu->arch.pause = false;
> > > swake_up(wq);
> > > }
> > > @@ -625,7 +624,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> > >
> > > update_vttbr(vcpu->kvm);
> > >
> > > - if (vcpu->arch.power_off || vcpu->arch.pause)
> > > + if (kvm_request_pending(vcpu)) {
> > > + if (kvm_check_request(KVM_REQ_VCPU_EXIT, vcpu)) {
> > > + if (vcpu->arch.pause)
> > > + vcpu_sleep(vcpu);
> > > + }
> >
> > Can we factor out this bit to a separate function,
> > kvm_handle_vcpu_requests() or something like that?
>
> Later patches make this look a bit better, but a function to bundle all
> the request handling up sounds good too. Will do.
>

Thanks,
-Christoffer