Re: [PATCH v3 07/20] virtio: add vhost-user-base and a generic vhost-user-device

[Alex Bennée]

Matias Ezequiel Vara Larsen <mvaralar@redhat.com> writes:

> On Mon, Jul 10, 2023 at 04:35:09PM +0100, Alex Bennée wrote:
>> In theory we shouldn't need to repeat so much boilerplate to support
>> vhost-user backends. This provides a generic vhost-user-base QOM
>> object and a derived vhost-user-device for which the user needs to
>> provide the few bits of information that aren't currently provided by
>> the vhost-user protocol. This should provide a baseline implementation
>> from which the other vhost-user stub can specialise.
>> 
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> 
>> ---
>> v2
>>   - split into vub and vud
<snip>
>> +
>> +    /*
>> +     * Disable guest notifiers, by default all notifications will be via the
>> +     * asynchronous vhost-user socket.
>> +     */
>> +    vdev->use_guest_notifier_mask = false;
>> +
>> +    /* Allocate queues */
>> +    vub->vqs = g_ptr_array_sized_new(vub->num_vqs);
>> +    for (int i = 0; i < vub->num_vqs; i++) {
>> +        g_ptr_array_add(vub->vqs,
>> +                        virtio_add_queue(vdev, 4, vub_handle_output));
>> +    }
>> +
>
> Hello Alex, apologies if someone already asked this. If I understand
> correctly, the second parameter of virtio_add_queue() is the len of the
> queue. Why have you chosen "4" as its value? Shall qemu query the len of
> the queue from the vhost-user device instead?

Hmm yeah that is inherited from the virtio-rng backend which has a
pretty short queue. I don't think it is intrinsic to the device
implementation (although I guess that depends if a device will have
multiple requests in flight).

I propose making is some useful ^2 (like 64) and adding a config knob to
increase it if needed.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro