Re: [virtio-dev] Re: [PATCH v1 6/6] vhost-user: add VFIO based accelerators support

[Jason Wang <jasowang@redhat.com>]

On 2018年01月26日 07:59, Michael S. Tsirkin wrote:
> > The virtual IOMMU isn't supported by the accelerators for now.
> > Because vhost-user currently lacks of an efficient way to share
> > the IOMMU table in VM to vhost backend. That's why the software
> > implementation of virtual IOMMU support in vhost-user backend
> > can't support dynamic mapping well.
> What exactly is meant by that? vIOMMU seems to work for people,
> it's not that fast if you change mappings all the time,
> but e.g. dpdk within guest doesn't.

Yes, software implementation support dynamic mapping for sure. I think 
the point is, current vhost-user backend can not program hardware IOMMU. 
So it can not let hardware accelerator to cowork with software vIOMMU. I 
think that's another call to implement the offloaded path inside qemu 
which has complete support for vIOMMU co-operated VFIO.

Thanks

> > Once this problem is solved
> > in vhost-user, virtual IOMMU can be supported by accelerators
> > too, and the IOMMU feature bit checking in this patch can be
> > removed.
> Given it works with software backends right now, I suspect
> this will be up to you guys to address.