Re: [PATCH] vhost: introduce vDPA based backend

[Jason Wang <jasowang@redhat.com>]

On 2020/2/5 下午5:23, Michael S. Tsirkin wrote:
> On Wed, Feb 05, 2020 at 03:50:14PM +0800, Jason Wang wrote:
>> 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.
>>
>>
>>> Devices that has other ways to do the DMA mapping will call the proprietary APIs.
>> To confirm, do you prefer:
>>
>> 1) map/unmap
>>
>> or
>>
>> 2) pass all maps at one time?
>>
>> Thanks
>>
>>
> I mean we really already have both right? ATM 1 is used with an iommu
> and 2 without. I guess we can also have drivers ask for either or both
> ...


Yes, that looks better.

Thanks