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! zhun...@gmail.com From: Stefan Hajnoczi Date: 2016-11-11 20:03 To: zhun...@gmail.com CC: qemu Subject: Re: Re: [Qemu-devel] virtIO question On Thu, Nov 10, 2016 at 08:16:38PM +0800, zhun...@gmail.com 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.
