linux-kernel - Re: [PATCH v5] vhost: introduce mdev based hardware backend

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Date:   Thu, 7 Nov 2019 12:08:08 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tiwei Bie <tiwei.bie@...el.com>
Cc:     "Michael S. Tsirkin" <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,
        lingshan.zhu@...el.com
Subject: Re: [PATCH v5] vhost: introduce mdev based hardware backend


On 2019/11/6 下午10:49, Tiwei Bie wrote:
>>>>>> +	default:
>>>>>> +		/*
>>>>>> +		 * VHOST_SET_MEM_TABLE, VHOST_SET_LOG_BASE, and
>>>>>> +		 * VHOST_SET_LOG_FD are not used yet.
>>>>>> +		 */
>>>>> If we don't even use them, there's probably no need to call
>>>>> vhost_dev_ioctl(). This may help to avoid confusion when we want to develop
>>>>> new API for e.g dirty page tracking.
>>>> Good point. It's better to reject these ioctls for now.
>>>>
>>>> PS. One thing I may need to clarify is that, we need the
>>>> VHOST_SET_OWNER ioctl to get the vq->handle_kick to work.
>>>> So if we don't call vhost_dev_ioctl(), we will need to
>>>> call vhost_dev_set_owner() directly.
>> I may miss something, it looks to me the there's no owner check in
>> vhost_vring_ioctl() and the vhost_poll_start() can make sure handle_kick
>> works?
> Yeah, there is no owner check in vhost_vring_ioctl().
> IIUC, vhost_poll_start() will start polling the file. And when
> event arrives, vhost_poll_wakeup() will be called, and it will
> queue work to work_list and wakeup worker to finish the work.
> And the worker is created by vhost_dev_set_owner().
>

Right, rethink about this. It looks to me we need:

- Keep VHOST_SET_OWNER, this could be used for future control vq where 
it needs a kthread to access the userspace memory

- Temporarily filter  SET_LOG_BASE and SET_LOG_FD until we finalize the 
API for dirty page tracking.

- For kick through kthread, it looks sub-optimal but we can address this 
in the future, e.g call handle_vq_kick directly in vhost_poll_queue 
(probably a flag for vhost_poll) and deal with the synchronization in 
vhost_poll_flush carefully.

Thanks