get_vring_base should not reset the queue

[germag]

Summary of the PR

The spec specifies that on receiving GET_VRING_BASE the backend should stop the vring, but not that it must be reset. This is intended for VHOST_USER_RESET_DEVICE, also in this case the spec makes a difference between stopping and disabling the ring.

The spec also doesn't forbid to send VHOST_USER_SET_VRING_ENABLE to enable the vring after receiving GET_VRING_BASE or sending more GET_VRING_BASE messages, which would always respond 0. Moreover, qemu[0] doesn't reset the vring either.

[0] https://github.com/qemu/qemu/blob/792f77f376adef944f9a03e601f6ad90c2f891b2/subprojects/libvhost-user/libvhost-user.c#L1175

Note:
I just added blank lines around self.vrings[index as usize].set_queue_ready(false); because the first time I worked on this (#154) I didn't realize that the ring was already being disabled.

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[germag]

In my previous PR (#154) I was wrong about DPDK[1], at the end it calls vring_invalidate(dev, vq); that does something similar to Queue::reset(), however, I still think it is not the right place to call reset, I think it is incorrectly assumed that GET_VRING_BASE will always be the last message.

[1] https://github.com/DPDK/dpdk/blob/d03446724972d2a1bb645ce7f3e64f5ef0203d61/lib/vhost/vhost_user.c#L2133

[stefano-garzarella]

In my previous PR (#154) I was wrong about DPDK[1], at the end it calls vring_invalidate(dev, vq); that does something similar to Queue::reset(),

Thanks for checking!

however, I still think it is not the right place to call reset, I think it is incorrectly assumed that GET_VRING_BASE will always be the last message.

Yep, I agree. I think multiple calls of GET_VRING_BASE should return the same index.
I think we should support VHOST_USER_RESET_DEVICE, in the meantime maybe a well place could be VHOST_USER_RESET_OWNER, which itself should be a NOP, but I see in QEMU that it is used to reset the device when VHOST_USER_PROTOCOL_F_RESET_DEVICE has not been negotiated.

[stefano-garzarella]

@jiangliu @sboeuf can you take a look?
And eventually test with your code to be sure we aren't breaking anything?