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

[Jason Wang <jasowang@redhat.com>]

On 2019/7/4 下午2:21, Tiwei Bie wrote:
> On Thu, Jul 04, 2019 at 12:31:48PM +0800, Jason Wang wrote:
>> On 2019/7/3 下午9:08, Tiwei Bie wrote:
>>> On Wed, Jul 03, 2019 at 08:16:23PM +0800, Jason Wang wrote:
>>>> On 2019/7/3 下午7:52, Tiwei Bie wrote:
>>>>> On Wed, Jul 03, 2019 at 06:09:51PM +0800, Jason Wang wrote:
>>>>>> On 2019/7/3 下午5:13, Tiwei Bie wrote:
>>>>>>> Details about this can be found here:
>>>>>>>
>>>>>>> https://lwn.net/Articles/750770/
>>>>>>>
>>>>>>> What's new in this version
>>>>>>> ==========================
>>>>>>>
>>>>>>> A new VFIO device type is introduced - vfio-vhost. This addressed
>>>>>>> some comments from here:https://patchwork.ozlabs.org/cover/984763/
>>>>>>>
>>>>>>> Below is the updated device interface:
>>>>>>>
>>>>>>> Currently, there are two regions of this device: 1) CONFIG_REGION
>>>>>>> (VFIO_VHOST_CONFIG_REGION_INDEX), which can be used to setup the
>>>>>>> device; 2) NOTIFY_REGION (VFIO_VHOST_NOTIFY_REGION_INDEX), which
>>>>>>> can be used to notify the device.
>>>>>>>
>>>>>>> 1. CONFIG_REGION
>>>>>>>
>>>>>>> The region described by CONFIG_REGION is the main control interface.
>>>>>>> Messages will be written to or read from this region.
>>>>>>>
>>>>>>> The message type is determined by the `request` field in message
>>>>>>> header. The message size is encoded in the message header too.
>>>>>>> The message format looks like this:
>>>>>>>
>>>>>>> struct vhost_vfio_op {
>>>>>>> 	__u64 request;
>>>>>>> 	__u32 flags;
>>>>>>> 	/* Flag values: */
>>>>>>>      #define VHOST_VFIO_NEED_REPLY 0x1 /* Whether need reply */
>>>>>>> 	__u32 size;
>>>>>>> 	union {
>>>>>>> 		__u64 u64;
>>>>>>> 		struct vhost_vring_state state;
>>>>>>> 		struct vhost_vring_addr addr;
>>>>>>> 	} payload;
>>>>>>> };
>>>>>>>
>>>>>>> The existing vhost-kernel ioctl cmds are reused as the message
>>>>>>> requests in above structure.
>>>>>> Still a comments like V1. What's the advantage of inventing a new protocol?
>>>>> I'm trying to make it work in VFIO's way..
>>>>>
>>>>>> I believe either of the following should be better:
>>>>>>
>>>>>> - using vhost ioctl,  we can start from SET_VRING_KICK/SET_VRING_CALL and
>>>>>> extend it with e.g notify region. The advantages is that all exist userspace
>>>>>> program could be reused without modification (or minimal modification). And
>>>>>> vhost API hides lots of details that is not necessary to be understood by
>>>>>> application (e.g in the case of container).
>>>>> Do you mean reusing vhost's ioctl on VFIO device fd directly,
>>>>> or introducing another mdev driver (i.e. vhost_mdev instead of
>>>>> using the existing vfio_mdev) for mdev device?
>>>> Can we simply add them into ioctl of mdev_parent_ops?
>>> Right, either way, these ioctls have to be and just need to be
>>> added in the ioctl of the mdev_parent_ops. But another thing we
>>> also need to consider is that which file descriptor the userspace
>>> will do the ioctl() on. So I'm wondering do you mean let the
>>> userspace do the ioctl() on the VFIO device fd of the mdev
>>> device?
>>>
>> Yes.
> Got it! I'm not sure what's Alex opinion on this. If we all
> agree with this, I can do it in this way.
>
>> Is there any other way btw?
> Just a quick thought.. Maybe totally a bad idea.


It's not for sure :)


>   I was thinking
> whether it would be odd to do non-VFIO's ioctls on VFIO's device
> fd. So I was wondering whether it's possible to allow binding
> another mdev driver (e.g. vhost_mdev) to the supported mdev
> devices. The new mdev driver, vhost_mdev, can provide similar
> ways to let userspace open the mdev device and do the vhost ioctls
> on it. To distinguish with the vfio_mdev compatible mdev devices,
> the device API of the new vhost_mdev compatible mdev devices
> might be e.g. "vhost-net" for net?
>
> So in VFIO case, the device will be for passthru directly. And
> in VHOST case, the device can be used to accelerate the existing
> virtualized devices.
>
> How do you think?


If my understanding is correct, there will be no VFIO ioctl if we go for 
vhost_mdev?

Thanks


>
> Thanks,
> Tiwei
>> Thanks
>>