Re: [PATCH v2] vhost: introduce mdev based hardware backend

[Jason Wang <jasowang@redhat.com>]

On 2019/10/23 下午3:07, Tiwei Bie wrote:
> On Wed, Oct 23, 2019 at 01:46:23PM +0800, Jason Wang wrote:
>> On 2019/10/23 上午11:02, Tiwei Bie wrote:
>>> On Tue, Oct 22, 2019 at 09:30:16PM +0800, Jason Wang wrote:
>>>> On 2019/10/22 下午5:52, Tiwei Bie wrote:
>>>>> This patch introduces a mdev based hardware vhost backend.
>>>>> This backend is built on top of the same abstraction used
>>>>> in virtio-mdev and provides a generic vhost interface for
>>>>> userspace to accelerate the virtio devices in guest.
>>>>>
>>>>> This backend is implemented as a mdev device driver on top
>>>>> of the same mdev device ops used in virtio-mdev but using
>>>>> a different mdev class id, and it will register the device
>>>>> as a VFIO device for userspace to use. Userspace can setup
>>>>> the IOMMU with the existing VFIO container/group APIs and
>>>>> then get the device fd with the device name. After getting
>>>>> the device fd of this device, userspace can use vhost ioctls
>>>>> to setup the backend.
>>>>>
>>>>> Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
>>>>> ---
>>>>> This patch depends on below series:
>>>>> https://lkml.org/lkml/2019/10/17/286
>>>>>
>>>>> v1 -> v2:
>>>>> - Replace _SET_STATE with _SET_STATUS (MST);
>>>>> - Check status bits at each step (MST);
>>>>> - Report the max ring size and max number of queues (MST);
>>>>> - Add missing MODULE_DEVICE_TABLE (Jason);
>>>>> - Only support the network backend w/o multiqueue for now;
>>>> Any idea on how to extend it to support devices other than net? I think we
>>>> want a generic API or an API that could be made generic in the future.
>>>>
>>>> Do we want to e.g having a generic vhost mdev for all kinds of devices or
>>>> introducing e.g vhost-net-mdev and vhost-scsi-mdev?
>>> One possible way is to do what vhost-user does. I.e. Apart from
>>> the generic ring, features, ... related ioctls, we also introduce
>>> device specific ioctls when we need them. As vhost-mdev just needs
>>> to forward configs between parent and userspace and even won't
>>> cache any info when possible,
>>
>> So it looks to me this is only possible if we expose e.g set_config and
>> get_config to userspace.
> The set_config and get_config interface isn't really everything
> of device specific settings. We also have ctrlq in virtio-net.


Yes, but it could be processed by the exist API. Isn't it? Just set ctrl 
vq address and let parent to deal with that.


>
>>
>>> I think it might be better to do
>>> this in one generic vhost-mdev module.
>>
>> Looking at definitions of VhostUserRequest in qemu, it mixed generic API
>> with device specific API. If we want go this ways (a generic vhost-mdev),
>> more questions needs to be answered:
>>
>> 1) How could userspace know which type of vhost it would use? Do we need to
>> expose virtio subsystem device in for userspace this case?
>>
>> 2) That generic vhost-mdev module still need to filter out unsupported
>> ioctls for a specific type. E.g if it probes a net device, it should refuse
>> API for other type. This in fact a vhost-mdev-net but just not modularize it
>> on top of vhost-mdev.
>>
>>
>>>>> - Some minor fixes and improvements;
>>>>> - Rebase on top of virtio-mdev series v4;
> [...]
>>>>> +
>>>>> +static long vhost_mdev_get_features(struct vhost_mdev *m, u64 __user *featurep)
>>>>> +{
>>>>> +	if (copy_to_user(featurep, &m->features, sizeof(m->features)))
>>>>> +		return -EFAULT;
>>>> As discussed in previous version do we need to filter out MQ feature here?
>>> I think it's more straightforward to let the parent drivers to
>>> filter out the unsupported features. Otherwise it would be tricky
>>> when we want to add more features in vhost-mdev module,
>>
>> It's as simple as remove the feature from blacklist?
> It's not really that easy. It may break the old drivers.


I'm not sure I understand here, we do feature negotiation anyhow. For 
old drivers do you mean the guest drivers without MQ?


>
>>
>>> i.e. if
>>> the parent drivers may expose unsupported features and relay on
>>> vhost-mdev to filter them out, these features will be exposed
>>> to userspace automatically when they are enabled in vhost-mdev
>>> in the future.
>>
>> The issue is, it's only that vhost-mdev knows its own limitation. E.g in
>> this patch, vhost-mdev only implements a subset of transport API, but parent
>> doesn't know about that.
>>
>> Still MQ as an example, there's no way (or no need) for parent to know that
>> vhost-mdev does not support MQ.
> The mdev is a MDEV_CLASS_ID_VHOST mdev device. When the parent
> is being developed, it should know the currently supported features
> of vhost-mdev.


How can parent know MQ is not supported by vhost-mdev?


>
>> And this allows old kenrel to work with new
>> parent drivers.
> The new drivers should provide things like VIRTIO_MDEV_F_VERSION_1
> to be compatible with the old kernels. When VIRTIO_MDEV_F_VERSION_1
> is provided/negotiated, the behaviours should be consistent.


To be clear, I didn't mean a change in virtio-mdev API, I meant:

1) old vhost-mdev kernel driver that filters out MQ

2) new parent driver that support MQ


>
>> So basically we have three choices here:
>>
>> 1) Implement what vhost-user did and implement a generic vhost-mdev (but may
>> still have lots of device specific code). To support advanced feature which
>> requires the access to config, still lots of API that needs to be added.
>>
>> 2) Implement what vhost-kernel did, have a generic vhost-mdev driver and a
>> vhost bus on top for match a device specific API e.g vhost-mdev-net. We
>> still have device specific API but limit them only to device specific
>> module. Still require new ioctls for advanced feature like MQ.
>>
>> 3) Simply expose all virtio-mdev transport to userspace.
> Currently, virtio-mdev transport is a set of function callbacks
> defined in kernel. How to simply expose virtio-mdev transport to
> userspace?


The most straightforward way is to have an 1:1 mapping between ioctl and 
virito_mdev_device_ops.

Thanks


>
>
>> A generic module
>> without any type specific code (like virtio-mdev). No need dedicated API for
>> e.g MQ. But then the API will look much different than current vhost did.
>>
>> Consider the limitation of 1) I tend to choose 2 or 3. What's you opinion?
>>
>>