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

[Tiwei Bie <tiwei.bie@intel.com>]

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.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.
> 
> 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