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

[Stefan Hajnoczi]

On Wed, Dec 15, 2021 at 11:18:05AM +0800, Jason Wang wrote:
> On Tue, Dec 14, 2021 at 9:11 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > On Tue, Dec 14, 2021 at 10:22:53AM +0800, Jason Wang wrote:
> > > On Mon, Dec 13, 2021 at 11:14 PM Stefan Hajnoczi <stefanha@redhat.com> 
> > > wrote:
> > > >
> > > > On Mon, Dec 13, 2021 at 10:47:00AM +0800, Jason Wang wrote:
> > > > > 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.
> > > >
> > > > That's an interesting idea. A generic vDPA VirtIODevice could exposed as
> > > >
> > > >   --device vhost-vdpa-pci,
> > > >            [vhostfd=FD,|
> > > >             vhostpath=/dev/vhost-vdpa-N]
> > > >
> > > > (and for virtio-mmio and virtio-ccw too).
> > > >
> > > > I don't think this is possible yet because the vhost_vdpa ioctls are
> > > > missing some introspection functionality. Here is what I found:
> > > > - Device ID: ok, use VHOST_VDPA_GET_DEVICE_ID
> > > > - Device feature bits: ok, use VHOST_GET_BACKEND_FEATURES
> > > > - Configuration space size: missing, need ioctl for 
> > > > ops->get_config_size()
> > >
> > > Any specific reason that we need this considering we've already had
> > > VHOST_VDPA_GET_CONFIG and we do the size validation there?
> >
> > QEMU's virtio_init() takes a size_t config_size argument. We need to
> > determine the size of the vhost_vdpa's configuration space in order to
> > create the VirtIODevice in QEMU.
> >
> > Do you mean probing by checking for the VHOST_VDPA_GET_CONFIG -E2BIG
> > return value? It's hacky but I guess it's possible to do a binary search
> > that calls VHOST_VDPA_GET_CONFIG each iteration and reduces the size if
> > -E2BIG is returned or increases the size otherwise.
> >
> > Or do you mean re-writing QEMU's hw/virtio/virtio.c to allow the
> > VirtIODevice to override the size and we pass accesses through to
> > vhost_vdpa. That way it might be possible to avoid fetching the
> > configuration space size at startup, but I'm not sure this will work
> > because QEMU might depend on knowing the exact size (e.g. live
> > migration).
> 
> Good point, so looking at virtio-blk it has:
> 
>     virtio_blk_set_config_size(s, s->host_features);
>     virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, s->config_size);
> 
> I think here virtio-blk/net should check the vhost-vdpa features here
> and fail if they are not the same?

The vhost feature bit code in QEMU is complicated and I can't respond
without investing too much time studying it :).

> This looks better than overriding the config_size with what vhost-vdpa
> provides since it can override the features that the cli tries to
> enable.

I'm thinking about the generic --device vhost-vdpa idea. QEMU should not
require knowledge of the device feature bits in that case, so it cannot
calculate the configuration space size.

> >
> > > > - Max virtqueue size: ok, VHOST_VDPA_GET_VRING_NUM
> > > > - Number of virtqueues: probe using VHOST_GET_VRING_BASE?
> > >
> > > I'm not sure whether or not we need this and it seems not necessary
> > > since it can be deduced from the config space and features.
> >
> > It can only be deduced in a device-specific way (net, blk, etc). I can't
> > think of a way to detect the number of virtqueues for an arbitrary
> > VIRTIO device from the features bits and configuration space contents.
> 
> Yes, I'm not against this idea but it looks to me it works even without this.
> 
> Modern PCI has num_queues but we don't have things like this in MMIO
> and legacy PCI.

Even if the VIRTIO hardware interface doesn't expose this information to
the guest, QEMU's VirtIODevice API needs it. Device emulation code must
call virtio_add_queue() to expose virtqueues to the guest.

I suppose --device vhost-vdpa could probe the number of virtqueues using
VHOST_GET_VRING_BASE and then call virtio_add_queue(), but it's a little
hacky and involves unnecessary ioctl calls.

Instead I would add ioctls to fetch the configuration space size and
number of virtqueues from the vhost_vdpa device.

With these two ioctls added --device vhost-vdpa could create a
VirtIODevice that works with any QEMU VIRTIO transport (pci, mmio, ccw).
It would work with any vDPA device without device-specific knowledge.
Live migration might be possible with some additional work.

Stefan

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