Re: [PATCH v2 4/9] KVM: arm/arm64: replace vcpu->arch.pause with a vcpu request

[Christoffer Dall <cdall@linaro.org>]

On Tue, Apr 04, 2017 at 08:15:09PM +0200, Paolo Bonzini wrote:
> 
> 
> On 04/04/2017 19:57, Christoffer Dall wrote:
> >> 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.
> > I see, that's why the cmpxchg() works the way it does.  We just still
> > need to move the vcpu->mode = IN_GUEST_MODE before our
> > with-interrupts-disabled check.
> > 
> > What I'm not sure is why you can get away without using a memory barrier
> > or WRITE_ONCE on x86, but is this simply because x86 is a strongly
> > ordered architecture?
> 
> x86 does have a memory barrier:
> 
>         vcpu->mode = IN_GUEST_MODE;
> 
>         srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>         smp_mb__after_srcu_read_unlock();

duh, the long complicated barrier version made me totally miss it.
Sorry.

> 
>         /*
>          * This handles the case where a posted interrupt was
>          * notified with kvm_vcpu_kick.
>          */
>         if (kvm_lapic_enabled(vcpu)) {
>                 if (kvm_x86_ops->sync_pir_to_irr && vcpu->arch.apicv_active)
>                         kvm_x86_ops->sync_pir_to_irr(vcpu);
>         }
> 
>         if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
> 
> and WRITE_ONCE is not needed if you have a memory barrier (though I find it
> more self-documenting to use it anyway).
> 

ok, thanks.

-Christoffer