Re: [PATCH] vhost: introduce mdev based hardware backend

From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Tiwei Bie <tiwei.bie@intel.com>,
	alex.williamson@redhat.com, maxime.coquelin@redhat.com,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, dan.daly@intel.com,
	cunming.liang@intel.com, zhihong.wang@intel.com,
	lingshan.zhu@intel.com
Subject: Re: [PATCH] vhost: introduce mdev based hardware backend
Date: Fri, 27 Sep 2019 21:17:56 +0800	[thread overview]
Message-ID: <b6f6ffb2-0b16-5041-be2e-94b805c6a4c9@redhat.com> (raw)
In-Reply-To: <20190927084408-mutt-send-email-mst@kernel.org>

On 2019/9/27 下午8:46, Michael S. Tsirkin wrote:
> On Fri, Sep 27, 2019 at 08:17:47PM +0800, Jason Wang wrote:
>> On 2019/9/27 下午5:41, Michael S. Tsirkin wrote:
>>> On Fri, Sep 27, 2019 at 11:27:12AM +0800, Jason Wang wrote:
>>>> On 2019/9/26 下午9:14, Tiwei Bie wrote:
>>>>> On Thu, Sep 26, 2019 at 04:35:18AM -0400, Michael S. Tsirkin wrote:
>>>>>> On Thu, Sep 26, 2019 at 12:54:27PM +0800, Tiwei Bie wrote:
>>>>> [...]
>>>>>>> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
>>>>>>> index 40d028eed645..5afbc2f08fa3 100644
>>>>>>> --- a/include/uapi/linux/vhost.h
>>>>>>> +++ b/include/uapi/linux/vhost.h
>>>>>>> @@ -116,4 +116,12 @@
>>>>>>>     #define VHOST_VSOCK_SET_GUEST_CID	_IOW(VHOST_VIRTIO, 0x60, __u64)
>>>>>>>     #define VHOST_VSOCK_SET_RUNNING		_IOW(VHOST_VIRTIO, 0x61, int)
>>>>>>> +/* VHOST_MDEV specific defines */
>>>>>>> +
>>>>>>> +#define VHOST_MDEV_SET_STATE	_IOW(VHOST_VIRTIO, 0x70, __u64)
>>>>>>> +
>>>>>>> +#define VHOST_MDEV_S_STOPPED	0
>>>>>>> +#define VHOST_MDEV_S_RUNNING	1
>>>>>>> +#define VHOST_MDEV_S_MAX	2
>>>>>>> +
>>>>>>>     #endif
>>>>>> So assuming we have an underlying device that behaves like virtio:
>>>>> I think they are really good questions/suggestions. Thanks!
>>>>>
>>>>>> 1. Should we use SET_STATUS maybe?
>>>>> I like this idea. I will give it a try.
>>>>>
>>>>>> 2. Do we want a reset ioctl?
>>>>> I think it is helpful. If we use SET_STATUS, maybe we
>>>>> can use it to support the reset.
>>>>>
>>>>>> 3. Do we want ability to enable rings individually?
>>>>> I will make it possible at least in the vhost layer.
>>>> Note the API support e.g set_vq_ready().
>>> virtio spec calls this "enabled" so let's stick to that.
>>
>> Ok.
>>
>>
>>>>>> 4. Does device need to limit max ring size?
>>>>>> 5. Does device need to limit max number of queues?
>>>>> I think so. It's helpful to have ioctls to report the max
>>>>> ring size and max number of queues.
>>>> An issue is the max number of queues is done through a device specific way,
>>>> usually device configuration space. This is supported by the transport API,
>>>> but how to expose it to userspace may need more thought.
>>>>
>>>> Thanks
>>> an ioctl for device config?  But for v1 I'd be quite happy to just have
>>> a minimal working device with 2 queues.
>>
>> I'm fully agree, and it will work as long as VIRTIO_NET_F_MQ and
>> VIRTIO_NET_F_CTRL_VQ is not advertised by the mdev device.
>>
>> Thanks
> Hmm this means we need to validate the features bits,
> not just pass them through to the hardware.
> Problem is, how do we add more feature bits later,
> without testing all hardware?
> I guess this means the device specific driver must do it.
>

That looks not good, maybe a virtio device id based features blacklist 
in vhost-mdev. Then MQ and CTRL_VQ could be filtered out by vhost-mdev.

Thanks

>>>>> Thanks!