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

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Date:   Thu, 4 Jul 2019 12:31:48 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>
Cc:     mst@...hat.com, alex.williamson@...hat.com,
        maxime.coquelin@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, dan.daly@...el.com,
        cunming.liang@...el.com, zhihong.wang@...el.com
Subject: Re: [RFC v2] vhost: introduce mdev based hardware vhost backend


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. Is there any other way btw?

Thanks