Re: [PATCH v2] vhost: Fix default value of kickfd and callfd

[Tetsuya Mukawa <mukawa@igel.co.jp>]

On 2016/03/11 16:19, Yuanhan Liu wrote:
> On Thu, Mar 10, 2016 at 04:06:05PM +0900, Tetsuya Mukawa wrote:
>> Currently, default values of kickfd and callfd are -1.
>> If the values are -1, current code guesses kickfd and callfd haven't
>> been initialized yet. And vhost library will guess the virtqueue isn't
>> ready for processing.
>> But callfd and kickfd will be set as -1 when "--enable-kvm"
>> isn't specified in QEMU command line. It means we cannot treat -1 as
>> uninitialized state. The patch changes default values to -2. And the
>> patch defines -2 as VIRTIO_UNINITIALIZED_EVENTFD.
> This looks more like a workaround to me. 


Hi Yuanhan,

Sorry for late reply.
I have checked QEMU documentation, and found below.

----------
 * VHOST_USER_SET_VRING_CALL

      Id: 14
      Equivalent ioctl: VHOST_SET_VRING_CALL
      Master payload: u64

      Set the event file descriptor to signal when buffers are used. It
      is passed in the ancillary data.
      Bits (0-7) of the payload contain the vring index. Bit 8 is the
      invalid FD flag.
----------

VHOST_USER_SET_VRING_KICK has almost same description.
I will check this invalid flag, and if it works for our case, then will
use it.
How about it?

> Besides, this patch would make
> following fail:
>
>     eventfd_write(vq->callfd, (eventfd_t)1);

It's my fault.
I thought above case should be blocked by virtio spec itself. So just
leave it.
But eventfd is came from vhost spec, so VRING_AVAIL_F_NO_INTERRUPT might
be set even when kickfd and callfd are -1.
Thanks for checking it.

Tetsuya

> 	--yliu