Re: [PART1 RFC v3 12/12] svm: Manage vcpu load/unload when enable AVIC

[Suravee Suthikulpanit]

Hi Radim,

On 3/31/16 21:19, Radim KrÄmÃÅ wrote:
> 2016-03-31 15:52+0700, Suravee Suthikulpanit:
> > On 03/19/2016 04:44 AM, Radim KrÄmÃÅ wrote:
> > > 2016-03-18 01:09-0500, Suravee Suthikulpanit:
> > > > +	} else {
> > > > +		new_entry = READ_ONCE(*entry);
> > > > +		/**
> > > > +		 * This handles the case when vcpu is scheduled out
> > > > +		 * and has not yet not called blocking. We save the
> > > > +		 * AVIC running flag so that we can restore later.
> > > > +		 */
> > >
> > > is_running must be disabled in between ...blocking and ...unblocking,
> > > because we don't want to miss interrupts and block forever.
> > > I somehow don't get it from the comment. :)
> >
> > Not sure if I understand your concern.  However, the is_running bit
> > setting/clearing should be handled in the avic_set_running below. This part
> > only handles othe case when the is_running bit still set when calling
> > vcpu_put (and later on loading some other vcpus). This way, when we are
> > re-loading this vcpu, we can restore the is_running bit accordingly.
>
> I think that the comment is misleading.  The saved is_running flag only
> matters after svm_vcpu_blocking, yet the comment says that it handles
> the irrelevant case before.

Actually, my understanding is if the svm_vcpu_blocking() is called, the 
is_running bit would have been cleared. At this point, if the vcpu is 
unloaded. We should not need to worry about it. Is that not the case here?

> Another minor bug is that was_running isn't initialized to 1, so we need
> to halt before is_running gets set.

Just to make sure, you are referring to the point where the is_running 
is not set for first time the vcpu is loaded?  If so, I agree. Thanks 
for the good catch.

> It would be clearer to toggle a flag in svm_vcpu_(un)blocking and set
> is_running = !is_blocking.

Not sure what you meant here. We are already setting/unsetting the 
is_running bit when vcpu is blocking/unblocking. Are you suggesting just 
simply move the current avic_set_running() into the svm_vcpu_blocking 
and svm_vcpu_unblocking()?


> Doing so will also be immeasurably faster,
> because avic_vcpu_load is called far more than svm_vcpu_(un)blocking.

Actually, this is not the same as handling normal vcpu blocking and 
unblocking, which we are already setting/un-setting the is_running bit 
in the avic_set_running(). The was_running should only be set to 1 if 
the vcpu is unloaded but has not yet calling halt.

Am I missing your points somehow?

Thanks,
Suravee