Re: [virtio-dev] Re: Virtio Spec clarification

[Paolo Bonzini <pbonzini@redhat.com>]

On 31/01/2017 19:27, Michael S. Tsirkin wrote:
> On Tue, Jan 31, 2017 at 01:13:37PM -0500, Karandeep Chahal wrote:
>> Hello Virtio Developers,
>>
>> I need some clarification about the virtio spec, and I was wondering if you
>> can help me with that.
>>
>> If VIRTIO_F_EVENT_IDX feature is negotiated, when is the driver supposed to
>> update the used_event? Is the driver supposed to do it before or after it
>> updates the avail_idx?
> 
> used_event is related to processing used ring entries.
> It requests an interrupt each time the used index will reach
> a specific value.
> After updating it, the spec suggests that the driver check that the
> index did not reach that value yet.
> 
>> The reason I am asking this question is because if the driver updates the
>> used_event after it updates the avail_idx, there is a possibility that the
>> device will process and complete the IO before the driver can update the
>> used_event. My device is constantly polling for changes to avail_idx, and
>> sometimes it does not interrupt the driver because the used_event has not
>> been updated by the driver yet.
>
> I think this should not be a problem if driver checks the
> used index after updating used_event.

You also need a memory barrier instruction after updating the used_event.

Paolo