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

Andrew Jones
2017-05-09T17:17:11Z

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.

>
> > 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.

>
> > 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.

>
> > + }
> > +
> > + if (vcpu->arch.power_off)
> > vcpu_sleep(vcpu);
> >
> > /*
> > --
> > 2.9.3
> >
> Thanks,
> -Christoffer

Thanks,
drew