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 20:18, Andrew Jones wrote:
>> 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.
>
> I was wondering about the justification of
> 'if (vcpu->mode == EXITING_GUEST_MODE)' in the x86 code, as it seemed
> redundant to me with the requests.  I'll have another think on it to see
> if request-less kicks can be satisfied in all cases by this, as long as we
> have the mode setting, barrier, mode checking order ensured in vcpu run.

Yes, this is the justification.  You should add that to
kvm_arch_vcpu_ioctl_run to close the race window (as well as the
kvm_request_pending, just for good measure).  These two are not really
optional, they are part of how kvm_vcpu_exiting_guest_mode and requests
are supposed to work.  kvm_vcpu_exiting_guest_mode is optional, but ARM
is using it and it's a pity to undo it.

Once you have done this, you can choose whether to use requests or not
for pause and poweroff, but I think it will not be necessary.

Paolo