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

Suravee Suthikulpanit
2016-04-05T10:08:13Z

Hi Radim,

On 3/31/16 21:19, Radim Krčmář wrote:
> 2016-03-31 15:52+0700, Suravee Suthikulpanit:
>> On 03/19/2016 04:44 AM, Radim Krčmář 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