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Subject: Re: [PATCH] vhost: introduce mdev based hardware backend
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Tiwei Bie <tiwei.bie@intel.com>, alex.williamson@redhat.com,
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        cunming.liang@intel.com, zhihong.wang@intel.com,
        lingshan.zhu@intel.com
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <1aba781c-d776-422d-2629-86b11990a9b5@redhat.com>
Date:   Sun, 29 Sep 2019 15:39:05 +0800
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On 2019/9/27 下午9:23, Michael S. Tsirkin wrote:
> On Fri, Sep 27, 2019 at 09:17:56PM +0800, Jason Wang wrote:
>> 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
> Two implementations of e.g. virtio net can have different
> features whitelisted.


I meant for kernel driver, we won't blacklist any feature, but for 
vhost-mdev, we need to do that.


>   So I think there's no way but let
> the driver do it. We should probably provide a standard place
> in the ops for driver to supply the whitelist, to make sure
> drivers don't forget.


For 'driver' do you mean userspace driver of  the mdev device?

Thanks


>