RE: [PATCH] vhost: introduce vDPA based backend

[Shahaf Shuler]

Wednesday, February 5, 2020 9:50 AM, Jason Wang:
> Subject: Re: [PATCH] vhost: introduce vDPA based backend
> On 2020/2/5 äå3:15, Shahaf Shuler wrote:
> > Wednesday, February 5, 2020 4:03 AM, Tiwei Bie:
> >> Subject: Re: [PATCH] vhost: introduce vDPA based backend
> >>
> >> On Tue, Feb 04, 2020 at 11:30:11AM +0800, Jason Wang wrote:
> >>> On 2020/1/31 äå11:36, Tiwei Bie wrote:
> >>>> This patch introduces a vDPA based vhost backend. This backend is
> >>>> built on top of the same interface defined in virtio-vDPA and
> >>>> provides a generic vhost interface for userspace to accelerate the
> >>>> virtio devices in guest.
> >>>>
> >>>> This backend is implemented as a vDPA device driver on top of the
> >>>> same ops used in virtio-vDPA. It will create char device entry
> >>>> named vhost-vdpa/$vdpa_device_index for userspace to use.
> Userspace
> >>>> can use vhost ioctls on top of this char device to setup the backend.
> >>>>
> >>>> Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
> > [...]
> >
> >>>> +static long vhost_vdpa_do_dma_mapping(struct vhost_vdpa *v) {
> >>>> +	/* TODO: fix this */
> >>>
> >>> Before trying to do this it looks to me we need the following during
> >>> the probe
> >>>
> >>> 1) if set_map() is not supported by the vDPA device probe the IOMMU
> >>> that is supported by the vDPA device
> >>> 2) allocate IOMMU domain
> >>>
> >>> And then:
> >>>
> >>> 3) pin pages through GUP and do proper accounting
> >>> 4) store GPA->HPA mapping in the umem
> >>> 5) generate diffs of memory table and using IOMMU API to setup the
> >>> dma mapping in this method
> >>>
> >>> For 1), I'm not sure parent is sufficient for to doing this or need
> >>> to introduce new API like iommu_device in mdev.
> >> Agree. We may also need to introduce something like the iommu_device.
> >>
> > Would it be better for the map/umnap logic to happen inside each device ?
> > Devices that needs the IOMMU will call iommu APIs from inside the driver
> callback.
> 
> 
> Technically, this can work. But if it can be done by vhost-vpda it will make the
> vDPA driver more compact and easier to be implemented.

Need to see the layering of such proposal but am not sure. 
Vhost-vdpa is generic framework, while the DMA mapping is vendor specific. 
Maybe vhost-vdpa can have some shared code needed to operate on iommu, so drivers can re-use it.  to me it seems simpler than exposing a new iommu device. 

> 
> 
> > Devices that has other ways to do the DMA mapping will call the
> proprietary APIs.
> 
> 
> To confirm, do you prefer:
> 
> 1) map/unmap

It is not only that. AFAIR there also flush and invalidate calls, right?

> 
> or
> 
> 2) pass all maps at one time?

To me this seems more straight forward. 
It is correct that under hotplug and large number of memory segments the driver will need to understand the diff (or not and just reload the new configuration). However, my assumption here is that memory hotplug is heavy flow anyway, and the driver extra cycles will not be that visible

> 
> Thanks
> 
> 
> >