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

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Message-ID: <146752f4-174c-c916-3682-b965b96d7872@redhat.com>
Date:   Tue, 29 Oct 2019 18:48:27 +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 v2] vhost: introduce mdev based hardware backend


On 2019/10/29 下午5:57, Tiwei Bie wrote:
> On Mon, Oct 28, 2019 at 11:50:49AM +0800, Jason Wang wrote:
>> On 2019/10/28 上午9:58, Tiwei Bie wrote:
>>> On Fri, Oct 25, 2019 at 08:16:26AM -0400, Michael S. Tsirkin wrote:
>>>> On Fri, Oct 25, 2019 at 05:54:55PM +0800, Jason Wang wrote:
>>>>> On 2019/10/24 下午6:42, Jason Wang wrote:
>>>>>> Yes.
>>>>>>
>>>>>>
>>>>>>>    And we should try to avoid
>>>>>>> putting ctrl vq and Rx/Tx vqs in the same DMA space to prevent
>>>>>>> guests having the chance to bypass the host (e.g. QEMU) to
>>>>>>> setup the backend accelerator directly.
>>>>>> That's really good point.  So when "vhost" type is created, parent
>>>>>> should assume addr of ctrl_vq is hva.
>>>>>>
>>>>>> Thanks
>>>>> This works for vhost but not virtio since there's no way for virtio kernel
>>>>> driver to differ ctrl_vq with the rest when doing DMA map. One possible
>>>>> solution is to provide DMA domain isolation between virtqueues. Then ctrl vq
>>>>> can use its dedicated DMA domain for the work.
>>> It might not be a bad idea to let the parent drivers distinguish
>>> between virtio-mdev mdevs and vhost-mdev mdevs in ctrl-vq handling
>>> by mdev's class id.
>> Yes, that should work, I have something probable better, see below.
>>
>>
>>>>> Anyway, this could be done in the future. We can have a version first that
>>>>> doesn't support ctrl_vq.
>>> +1, thanks
>>>
>>>>> Thanks
>>>> Well no ctrl_vq implies either no offloads, or no XDP (since XDP needs
>>>> to disable offloads dynamically).
>>>>
>>>>          if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS)
>>>>              && (virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_TSO4) ||
>>>>                  virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_TSO6) ||
>>>>                  virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_ECN) ||
>>>>                  virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_UFO) ||
>>>>                  virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_CSUM))) {
>>>>                  NL_SET_ERR_MSG_MOD(extack, "Can't set XDP while host is implementing LRO/CSUM, disable LRO/CSUM first");
>>>>                  return -EOPNOTSUPP;
>>>>          }
>>>>
>>>> neither is very attractive.
>>>>
>>>> So yes ok just for development but we do need to figure out how it will
>>>> work down the road in production.
>>> Totally agree.
>>>
>>>> So really this specific virtio net device does not support control vq,
>>>> instead it supports a different transport specific way to send commands
>>>> to device.
>>>>
>>>> Some kind of extension to the transport? Ideas?
>> So it's basically an issue of isolating DMA domains. Maybe we can start with
>> transport API for querying per vq DMA domain/ASID?
>>
>> - for vhost-mdev, userspace can query the DMA domain for each specific
>> virtqueue. For control vq, mdev can return id for software domain, for the
>> rest mdev will return id of VFIO domain. Then userspace know that it should
>> use different API for preparing the virtqueue, e.g for vq other than control
>> vq, it should use VFIO DMA API. The control vq it should use hva instead.
>>
>> - for virito-mdev, we can introduce per-vq DMA device, and route DMA mapping
>> request for control vq back to mdev instead of the hardware. (We can wrap
>> them into library or helpers to ease the development of vendor physical
>> drivers).
> Thanks for this proposal! I'm thinking about it these days.
> I think it might be too complicated. I'm wondering whether we
> can have something simpler. I will post a RFC patch to show
> my idea today.


Thanks, will check.

Btw, for virtio-mdev, the change should be very minimal, will post an
RFC as well. For vhost-mdev, it could be just a helper to return an ID
for DMA domain like ID_VFIO or ID_HVA.

Or a more straightforward way is to force queues like control vq to use PA.


>
> Thanks,
> Tiwei
>