Date:   Thu, 17 Oct 2019 09:42:53 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefan Hajnoczi <stefanha@...il.com>
Cc:     kvm@...r.kernel.org, linux-s390@...r.kernel.org,
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Subject: Re: [PATCH V3 0/7] mdev based hardware virtio offloading support


On 2019/10/15 下午10:37, Stefan Hajnoczi wrote:
> On Tue, Oct 15, 2019 at 11:37:17AM +0800, Jason Wang wrote:
>> On 2019/10/15 上午1:49, Stefan Hajnoczi wrote:
>>> On Fri, Oct 11, 2019 at 04:15:50PM +0800, Jason Wang wrote:
>>>> There are hardware that can do virtio datapath offloading while having
>>>> its own control path. This path tries to implement a mdev based
>>>> unified API to support using kernel virtio driver to drive those
>>>> devices. This is done by introducing a new mdev transport for virtio
>>>> (virtio_mdev) and register itself as a new kind of mdev driver. Then
>>>> it provides a unified way for kernel virtio driver to talk with mdev
>>>> device implementation.
>>>>
>>>> Though the series only contains kernel driver support, the goal is to
>>>> make the transport generic enough to support userspace drivers. This
>>>> means vhost-mdev[1] could be built on top as well by resuing the
>>>> transport.
>>>>
>>>> A sample driver is also implemented which simulate a virito-net
>>>> loopback ethernet device on top of vringh + workqueue. This could be
>>>> used as a reference implementation for real hardware driver.
>>>>
>>>> Consider mdev framework only support VFIO device and driver right now,
>>>> this series also extend it to support other types. This is done
>>>> through introducing class id to the device and pairing it with
>>>> id_talbe claimed by the driver. On top, this seris also decouple
>>>> device specific parents ops out of the common ones.
>>> I was curious so I took a quick look and posted comments.
>>>
>>> I guess this driver runs inside the guest since it registers virtio
>>> devices?
>>
>> It could run in either guest or host. But the main focus is to run in the
>> host then we can use virtio drivers in containers.
>>
>>
>>> If this is used with physical PCI devices that support datapath
>>> offloading then how are physical devices presented to the guest without
>>> SR-IOV?
>>
>> We will do control path meditation through vhost-mdev[1] and vhost-vfio[2].
>> Then we will present a full virtio compatible ethernet device for guest.
>>
>> SR-IOV is not a must, any mdev device that implements the API defined in
>> patch 5 can be used by this framework.
> What I'm trying to understand is: if you want to present a virtio-pci
> device to the guest (e.g. using vhost-mdev or vhost-vfio), then how is
> that related to this patch series?


This series introduce some infrastructure that would be used by vhost-mdev:

1) allow new type of mdev devices/drivers other than vfio (through 
class_id and device ops)

2) a set of virtio specific callbacks that will be used by both 
vhost-mdev and virtio-mdev defined in patch 5

Then vhost-mdev can be implemented on top: a new mdev class id but reuse 
the callback defined in 2. Through this way the parent can provides a 
single set of callbacks (device ops) for both kernel virtio driver 
(through virtio-mdev) or userspace virtio driver (through vhost-mdev).


>
> Does this mean this patch series is useful mostly for presenting virtio
> devices to containers or the host?


Patch 6 is mainly for bare metal or container use case, through it could 
be used in guest as well. Patch 7 is a sample virtio mdev device 
implementation. Patch 1 - 5 was the infrastructure for implementing 
types other than vfio, the first user is virito-mdev, then Tiwei's 
vhost-mdev and Parav's mlx5 mdev.

Thanks


>
> Stefan

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