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Subject: Re: [PATCH] vhost: introduce mdev based hardware backend
To:     Tiwei Bie <tiwei.bie@intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     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
References: <20190926045427.4973-1-tiwei.bie@intel.com>
 <20190926042156-mutt-send-email-mst@kernel.org> <20190926131439.GA11652@___>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <8ab5a8d9-284d-bba5-803d-08523c0814e1@redhat.com>
Date:   Fri, 27 Sep 2019 11:27:12 +0800
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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().


>
>> 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


>
> Thanks!