Hi Jason, On Mon, Oct 21, 2019 at 01:48:53PM +0800, Jason Wang wrote: > On 2019/10/19 上午4:58, Will Deacon wrote: > > Staring at the code some more, my best bet at the moment > > is that the load of 'indirect->addr' is probably the one to worry about, > > since it's part of the vring and can be updated concurrently. > > I'm also confused about the barrier here, basically in driver side we did: > > 1) allocate pages > > 2) store pages in indirect->addr > > 3) smp_wmb() > > 4) increase the avail idx (somehow a tail pointer of vring) > > in vhost we did: > > 1) read avail idx > > 2) smp_rmb() > > 3) read indirect->addr > > 4) read from indirect->addr > > It looks to me even the data dependency barrier is not necessary since we > have rmb() which is sufficient for us to the correct indirect->addr and > driver are not expected to do any writing to indirect->addr after avail idx > is increased ?
That makes sense to me: the synchronization is done via vq->avail_idx() and so the subsequent access of the indirect pages doesn't need additional barriers, even on Alpha. Thanks. I'll write up a patch and adopt your explanation above. Will _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
