Re: [PATCH] vhost: introduce vDPA based backend

[Jason Wang <jasowang@redhat.com>]

On 2020/2/5 下午6:33, Michael S. Tsirkin wrote:
> On Wed, Feb 05, 2020 at 09:30:14AM +0000, Shahaf Shuler wrote:
>> Wednesday, February 5, 2020 9:50 AM, Jason Wang:
>>> Subject: Re: [PATCH] vhost: introduce vDPA based backend
>>> On 2020/2/5 下午3:15, 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 上午11:36, 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@intel.com>
>>>> [...]
>>>>
>>>>>>> +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
> I think we can just allow both, after all vhost already has both interfaces ...
> We just need a flag that tells userspace whether it needs to
> update all maps aggressively or can wait for a fault.


It looks to me such flag is not a must and we can introduce it later 
when device support page fault.

Thanks