Re: [Qemu-devel] virtIO question

[jack]

   thanks,but i really do not found when the driver will add mutiple
   buffers then call kick function,this is the key problem .

   æ�¥è‡ª é…æ—� MX5

   -------- 原始邮件 --------
   �件人：Stefan Hajnoczi <address@hidden>
   时间：周五 11月11日 20:03
   address@hidden
   抄�：qemu <address@hidden>
   主题：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 noti�cations 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.

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