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

[Paolo Bonzini <pbonzini@redhat.com>]

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();

        /*
         * 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).

Paolo