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.
signature.asc
Description: PGP signature
