On Wed, Apr 11, 2018 at 08:37:17PM +0300, Michael S. Tsirkin wrote: > On Wed, Apr 11, 2018 at 04:38:53PM +0800, Tiwei Bie wrote: > > On Wed, Apr 11, 2018 at 04:01:19PM +0800, Jason Wang wrote: > > > On 2018年04月11日 15:20, Tiwei Bie wrote: > > > > This patch introduces VHOST_USER_PROTOCOL_F_NEED_ALL_IOTLB > > > > feature for vhost-user. By default, vhost-user backend needs > > > > to query the IOTLBs from QEMU after meeting unknown IOVAs. > > > > With this protocol feature negotiated, QEMU will provide all > > > > the IOTLBs to vhost-user backend without waiting for the > > > > queries from backend. This is helpful when using a hardware > > > > accelerator which is not able to handle unknown IOVAs at the > > > > vhost-user backend. > > > > > > > > Signed-off-by: Tiwei Bie<tiwei....@intel.com> > > > > --- > > > > The idea of this patch is to let QEMU push all the IOTLBs > > > > to vhost-user backend without waiting for the queries from > > > > the backend. Because hardware accelerator at the vhost-user > > > > backend may not be able to handle unknown IOVAs. > > > > > > > > This is just a RFC for now. It seems that, it doesn't work > > > > as expected when guest is using kernel driver (To handle > > > > this case, it seems that some RAM regions' events also need > > > > to be listened). Any comments would be appreciated! Thanks! > > > > > > Interesting, a quick question is why this is needed? Can we just use exist > > > IOTLB update message? > > > > Yeah, we are still using the existing IOTLB update messages > > to send the IOTLB messages to backend. The only difference > > is that, QEMU won't wait for the queries before sending the > > IOTLB update messages. > > So I have a concern with that, in that without any flow > control the socket buffer used by vhost-user might become > full.
Each IOTLB update message needs a reply. So I think it won't happen. Best regards, Tiwei Bie > > We don't currently expect that. > > > Again, my understanding is that the biggest benefit from use of a > hardware accelerator is when notifications can be passed-through to the > guest. > > And since that involves VFIO, and since VFIO already needs to support > all kinds of IOMMUs, one wonders whether one can just pass that directly > to the VFIO instead of shipping it to vhost-user. > > > > > > > > It looks to me at least kernel does not need this. > > > > Something similar in kernel vhost is that, for kernel vhost, > > QEMU needs to push the IOTLBs of some ring addrs to kernel > > vhost backend without waiting for the queries. > > > > Best regards, > > Tiwei Bie > > That's really to work around a bug in kernel, we keep this around since > we want to support old kernels. > > > > > > > Thanks
