Re: [Qemu-devel] virtIO question

[address@hidden]

Hello，teacher,I may be found where the driver add mutiple buffer to virtqueue 
and then kick qemu side. It is when driver use NAPI to poll the device to get 
buffers,and it is in receive queue.but in transmit queue,every time driver add 
a buffer to virtqueue,then kick function is called!!!why ??is qemu handle 
buffer faster than driver add it??

thank you very much!



address@hidden
 
From: Stefan Hajnoczi
Date: 2016-11-11 20:03
To: address@hidden
CC: qemu
Subject: Re: Re: [Qemu-devel] virtIO question
On Thu, Nov 10, 2016 at 08:16:38PM +0800, address@hidden wrote:
> From this point of view ,I think it make sense well, thank you very much!
>  but I have another question about notify mechanism between virtIO driver and 
> qemu.
> according the source code of Linux and qemu,
> when driver add a sg buffer to send queue named sq,
> sq->vq->vring.avail->idx++
> vq->num_added++
> and then use virtqueue_kick_prepare to make sure if need notify qemu.
> it (new_idx-event_idx)<(new_idx-old_idx)
 
This expression is wrong.  The specification and Linux code both say:
 
  (u16)(new_idx - event_idx - 1) < (u16)(new_idx - old_idx)
 
Both the (u16) and the -1 matter.  Maybe that's why you are confused by
this?
 
> if it is true,then notify other side.
> However,every time driver add a sg,then virtqueue_kick_prepare is called,and 
> vq->num_added  is reseted to 0,so in fact ,I think vq->num_added is always 0 
> or 1。
 
A driver may add multiple buffers to the virtqueue by calling
virtqueue_add_sgs() or similar functions multiple times before kicking.
Therefore vq->num_added > 1 is possible.
 
> as to qemu side,every time when pop a elem from virtqueue,it set 
> VRingUsed.ring[vring.num] to the lastest VRingAvail.idx, this according the 
> arithmetic ((new_idx-event_idx)<(new_idx-old_idx)),it seems that this 
> mechanism does not make sense
 
You are basically asking "how does event_idx work?".  The specification
says:
 
  "The driver can ask the device to delay interrupts until an entry with
  an index specified by the “used_event” field is written in the used ring
  (equivalently, until the idx field in the used ring will reach the
  value used_event + 1)."
 
and:
 
  "The device can ask the driver to delay notifications until an entry
  with an index specified by the “avail_event” field is written in the
  available ring (equivalently, until the idx field in the used ring will
  reach the value avail_event + 1)."
 
Whenever the device or driver wants to notify, it first checks if the
index update crossed the event index set by the other side.