Re: [RFC] vhost-vdpa-net: add vhost-vdpa-net host device support

[Jason Wang]

On Sun, Dec 12, 2021 at 5:30 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Sat, Dec 11, 2021 at 03:00:27AM +0000, Longpeng (Mike, Cloud 
> Infrastructure Service Product Dept.) wrote:
> >
> >
> > > -----Original Message-----
> > > From: Stefan Hajnoczi [mailto:stefanha@redhat.com]
> > > Sent: Thursday, December 9, 2021 5:17 PM
> > > To: Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
> > > <longpeng2@huawei.com>
> > > Cc: jasowang@redhat.com; mst@redhat.com; parav@nvidia.com;
> > > xieyongji@bytedance.com; sgarzare@redhat.com; Yechuan 
> > > <yechuan@huawei.com>;
> > > Gonglei (Arei) <arei.gonglei@huawei.com>; qemu-devel@nongnu.org
> > > Subject: Re: [RFC] vhost-vdpa-net: add vhost-vdpa-net host device support
> > >
> > > On Wed, Dec 08, 2021 at 01:20:10PM +0800, Longpeng(Mike) wrote:
> > > > From: Longpeng <longpeng2@huawei.com>
> > > >
> > > > Hi guys,
> > > >
> > > > This patch introduces vhost-vdpa-net device, which is inspired
> > > > by vhost-user-blk and the proposal of vhost-vdpa-blk device [1].
> > > >
> > > > I've tested this patch on Huawei's offload card:
> > > > ./x86_64-softmmu/qemu-system-x86_64 \
> > > >     -device vhost-vdpa-net-pci,vdpa-dev=/dev/vhost-vdpa-0
> > > >
> > > > For virtio hardware offloading, the most important requirement for us
> > > > is to support live migration between offloading cards from different
> > > > vendors, the combination of netdev and virtio-net seems too heavy, we
> > > > prefer a lightweight way.
> > > >
> > > > Maybe we could support both in the future ? Such as:
> > > >
> > > > * Lightweight
> > > >  Net: vhost-vdpa-net
> > > >  Storage: vhost-vdpa-blk
> > > >
> > > > * Heavy but more powerful
> > > >  Net: netdev + virtio-net + vhost-vdpa
> > > >  Storage: bdrv + virtio-blk + vhost-vdpa
> > > >
> > > > [1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg797569.html
> > >
> > > Stefano presented a plan for vdpa-blk at KVM Forum 2021:
> > > https://kvmforum2021.sched.com/event/ke3a/vdpa-blk-unified-hardware-and-sof
> > > tware-offload-for-virtio-blk-stefano-garzarella-red-hat
> > >
> > > It's closer to today's virtio-net + vhost-net approach than the
> > > vhost-vdpa-blk device you have mentioned. The idea is to treat vDPA as
> > > an offload feature rather than a completely separate code path that
> > > needs to be maintained and tested. That way QEMU's block layer features
> > > and live migration work with vDPA devices and re-use the virtio-blk
> > > code. The key functionality that has not been implemented yet is a "fast
> > > path" mechanism that allows the QEMU virtio-blk device's virtqueue to be
> > > offloaded to vDPA.
> > >
> > > The unified vdpa-blk architecture should deliver the same performance
> > > as the vhost-vdpa-blk device you mentioned but with more features, so I
> > > wonder what aspects of the vhost-vdpa-blk idea are important to you?
> > >
> > > QEMU already has vhost-user-blk, which takes a similar approach as the
> > > vhost-vdpa-blk device you are proposing. I'm not against the
> > > vhost-vdpa-blk approach in priciple, but would like to understand your
> > > requirements and see if there is a way to collaborate on one vdpa-blk
> > > implementation instead of dividing our efforts between two.
> > >
> >
> > We prefer a simple way in the virtio hardware offloading case, it could 
> > reduce
> > our maintenance workload, we no need to maintain the virtio-net, netdev,
> > virtio-blk, bdrv and ... any more. If we need to support other vdpa devices
> > (such as virtio-crypto, virtio-fs) in the future, then we also need to 
> > maintain
> > the corresponding device emulation code?
> >
> > For the virtio hardware offloading case, we usually use the vfio-pci 
> > framework,
> > it saves a lot of our maintenance work in QEMU, we don't need to touch the 
> > device
> > types. Inspired by Jason, what we really prefer is "vhost-vdpa-pci/mmio", 
> > use it to
> > instead of the vfio-pci, it could provide the same performance as vfio-pci, 
> > but it's
> > *possible* to support live migrate between offloading cards from different 
> > vendors.
>
> OK, so the features you are dropping would be migration between
> a vdpa, vhost and virtio backends. I think given vhost-vdpa-blk is seems
> fair enough... What do others think?

I think it should be fine, and it would be even better to make it not
specific to device type.

Thanks

>
> > > Stefan
>