[RFC PATCH] KVM: Only register preempt notifiers and load arch cpu state as needed

[pbonzini@redhat.com (Paolo Bonzini)]

On 23/11/2017 18:06, Christoffer Dall wrote:
> On Thu, Nov 23, 2017 at 05:17:00PM +0100, Paolo Bonzini wrote:
>> On 23/11/2017 17:05, Christoffer Dall wrote:
>>> For example,
>>> arm64 is about to do significant work in vcpu load/put when running a
>>> vcpu, but not when doing things like KVM_SET_ONE_REG or
>>> KVM_SET_MP_STATE.
>>
>> Out of curiosity, in what circumstances are these ioctls a hot path?
>> Especially KVM_SET_MP_STATE.
> 
> Perhaps my commit message was misleading; we only want to do that for
> KVM_RUN, and not for anything else.  We're already doing things like
> potentially jumping to hyp mode and flushing VMIDs which really
> shouldn't be done unless we actually plan on running a VCPU, and we're
> going to do things like setting up the timer to handle timer interrupts
> in an ISR, which doesn't make sense unless the VCPU is running.
> 
> Add to that, that loading an entire VM's state onto hardware, only to
> read back a single register from hardware and returning it to user
> space, doesn't really fall within optimization vs. non-optimization in
> the critical path, but is just wrong, IMHO.

For GET/SET_ONE_REG it certainly makes sense.  For everything else, I'm
wondering which ioctls (and how many calls to each of them) exactly you
are seeing, and also on which userspace paths.

> That doesn't solve my need as I want to *only* do the arch vcpu_load for
> KVM_RUN, I should have been more clear in the commit message.

That's what you want to do, but it might not be what you need to do.

Paolo