Re: [Qemu-devel] [RFC] vhost-user: introduce F_NEED_ALL_IOTLB protocol feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 11, 2018 at 09:41:05PM +0800, Jason Wang wrote:
> 
> 
> On 2018年04月11日 16:38, 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.bie@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.
> 
> Yes, my question is not very clear. I mean why must need a new feature bit?
> It looks to me qemu code can work without this.
> 
> Thanks

Generally we avoid adding new messages without a protocol feature bit.
While careful analysis might sometimes prove it's not a strict
requirement, it's just overall a clean and robust approach.

> > 
> > > 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
> > 
> > > Thanks