Re: [PATCH v2] vhost: introduce mdev based hardware backend

[Jason Wang]

On 2019/10/24 äå4:03, Jason Wang wrote:
> On 2019/10/24 äå12:21, Tiwei Bie wrote:
> > On Wed, Oct 23, 2019 at 06:29:21PM +0800, Jason Wang wrote:
> > > On 2019/10/23 äå6:11, Tiwei Bie wrote:
> > > > On Wed, Oct 23, 2019 at 03:25:00PM +0800, Jason Wang wrote:
> > > > > On 2019/10/23 äå3:07, Tiwei Bie wrote:
> > > > > > On Wed, Oct 23, 2019 at 01:46:23PM +0800, Jason Wang wrote:
> > > > > > > On 2019/10/23 äå11:02, Tiwei Bie wrote:
> > > > > > > > On Tue, Oct 22, 2019 at 09:30:16PM +0800, Jason Wang wrote:
> > > > > > > > > On 2019/10/22 äå5:52, Tiwei Bie wrote:
> > > > > > > > > > This patch introduces a mdev based hardware vhost backend.
> > > > > > > > > > This backend is built on top of the same abstraction used
> > > > > > > > > > in virtio-mdev and provides a generic vhost interface for
> > > > > > > > > > userspace to accelerate the virtio devices in guest.
> > > > > > > > > >
> > > > > > > > > > This backend is implemented as a mdev device driver on top
> > > > > > > > > > of the same mdev device ops used in virtio-mdev but using
> > > > > > > > > > a different mdev class id, and it will register the device
> > > > > > > > > > as a VFIO device for userspace to use. Userspace can setup
> > > > > > > > > > the IOMMU with the existing VFIO container/group APIs and
> > > > > > > > > > then get the device fd with the device name. After getting
> > > > > > > > > > the device fd of this device, userspace can use vhost ioctls
> > > > > > > > > > to setup the backend.
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
> > > > > > > > > > ---
> > > > > > > > > > This patch depends on below series:
> > > > > > > > > > https://lkml.org/lkml/2019/10/17/286
> > > > > > > > > >
> > > > > > > > > > v1 -> v2:
> > > > > > > > > > - Replace _SET_STATE with _SET_STATUS (MST);
> > > > > > > > > > - Check status bits at each step (MST);
> > > > > > > > > > - Report the max ring size and max number of queues (MST);
> > > > > > > > > > - Add missing MODULE_DEVICE_TABLE (Jason);
> > > > > > > > > > - Only support the network backend w/o multiqueue for now;
> > > > > > > > > Any idea on how to extend it to support devices other than 
> > > > > > > > > net? I think we
> > > > > > > > > want a generic API or an API that could be made generic in the 
> > > > > > > > > future.
> > > > > > > > >
> > > > > > > > > Do we want to e.g having a generic vhost mdev for all kinds of 
> > > > > > > > > devices or
> > > > > > > > > introducing e.g vhost-net-mdev and vhost-scsi-mdev?
> > > > > > > > One possible way is to do what vhost-user does. I.e. Apart from
> > > > > > > > the generic ring, features, ... related ioctls, we also introduce
> > > > > > > > device specific ioctls when we need them. As vhost-mdev just needs
> > > > > > > > to forward configs between parent and userspace and even won't
> > > > > > > > cache any info when possible,
> > > > > > > So it looks to me this is only possible if we expose e.g 
> > > > > > > set_config and
> > > > > > > get_config to userspace.
> > > > > > The set_config and get_config interface isn't really everything
> > > > > > of device specific settings. We also have ctrlq in virtio-net.
> > > > > Yes, but it could be processed by the exist API. Isn't it? Just 
> > > > > set ctrl vq
> > > > > address and let parent to deal with that.
> > > > I mean how to expose ctrlq related settings to userspace?
> > >
> > > I think it works like:
> > >
> > > 1) userspace find ctrl_vq is supported
> > >
> > > 2) then it can allocate memory for ctrl vq and set its address through
> > > vhost-mdev
> > >
> > > 3) userspace can populate ctrl vq itself
> > I see. That is to say, userspace e.g. QEMU will program the
> > ctrl vq with the existing VHOST_*_VRING_* ioctls, and parent
> > drivers should know that the addresses used in ctrl vq are
> > host virtual addresses in vhost-mdev's case.
>
>
> That's really good point. And that means parent needs to differ vhost 
> from virtio. It should work. 


HVA may only work when we have something similar to VHOST_SET_OWNER 
which can reuse MM of its owner.


> But is there any chance to use DMA address? I'm asking since the API 
> then tends to be device specific. 


I wonder whether we can introduce MAP IOMMU notifier and get DMA 
mappings from that.

Thanks