On Mon, Dec 20, 2021 at 10:48:09AM +0800, Jason Wang wrote: > On Fri, Dec 17, 2021 at 4:35 PM Stefan Hajnoczi <stefa...@redhat.com> wrote: > > > > On Fri, Dec 17, 2021 at 12:26:53PM +0800, Jason Wang wrote: > > > > Dave: You created the VIRTIO vmstate infrastructure in QEMU. Please see > > the bottom of this email about moving to a standard VIRTIO device > > save/load format defined by the VIRTIO spec in the future. > > > > > On Thu, Dec 16, 2021 at 5:10 PM Stefan Hajnoczi <stefa...@redhat.com> > > > wrote: > > > > > > > > On Thu, Dec 16, 2021 at 11:01:40AM +0800, Jason Wang wrote: > > > > > On Wed, Dec 15, 2021 at 6:07 PM Stefan Hajnoczi <stefa...@redhat.com> > > > > > wrote: > > > > > > > > > > > > On Wed, Dec 15, 2021 at 11:18:05AM +0800, Jason Wang wrote: > > > > > > > On Tue, Dec 14, 2021 at 9:11 PM Stefan Hajnoczi > > > > > > > <stefa...@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 > > > > > > > > > <stefa...@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 > > > > > > > > > > > <m...@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:stefa...@redhat.com] > > > > > > > > > > > > > > Sent: Thursday, December 9, 2021 5:17 PM > > > > > > > > > > > > > > To: Longpeng (Mike, Cloud Infrastructure Service > > > > > > > > > > > > > > Product Dept.) > > > > > > > > > > > > > > <longpe...@huawei.com> > > > > > > > > > > > > > > Cc: jasow...@redhat.com; m...@redhat.com; > > > > > > > > > > > > > > pa...@nvidia.com; > > > > > > > > > > > > > > xieyon...@bytedance.com; sgarz...@redhat.com; > > > > > > > > > > > > > > Yechuan <yech...@huawei.com>; > > > > > > > > > > > > > > Gonglei (Arei) <arei.gong...@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 <longpe...@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. > > > > > > > > > > In this case, it looks to me the config size could be deduced from > > > > > VHOST_VDPA_GET_FEATURES? > > > > > > > > I think we're talking about different things, see below... > > > > > > > > > > > > > > > > > > > > > > > > > > > > - 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. > > > > > > > > > > We don't need this for current multiqueue virtio-net with vhost-vdpa > > > > > since the queue num were deduced from the VHOST_VDPA_GET_CONFIG during > > > > > the initialization of vhost-vdpa backend. > > > > > > > > > > If we are talking about generic vhost-vdpa-pci, we don't need > > > > > virtio_add_queue() in this case. > > > > > > > > When I say --device vhost-vdpa I mean a VirtIODevice in QEMU that takes > > > > any /dev/vhost-vdpa-N and exposes the device to the guest (over > > > > virtio-pci, virtio-mmio, or virtio-ccw). It's generic because it has no > > > > knowledge of specific device types. This means new device types can be > > > > added without modifying QEMU. > > > > > > > > I think the model you are describing is not generic because it relies on > > > > knowledge of specific device types (net, blk, scsi, etc) so it can > > > > interpret feature bits and configuration space fields. > > > > > > Yes, but what I meant is that in this case qemu can simply relay the > > > set/get config to vhost-vdpa. And the guest driver can enumerate the > > > number of queues correctly depending on his own knowledge. > > > > That requires changes to how virtqueues are managed by > > hw/virtio/virtio.c because today the code assumes QEMU knows the number > > of virtqueues. virtio_add_queue() must be called by device emulation > > before the guest driver can configure a virtqueue. > > Right. > > > > > > > > > > > When you originally said "it would be even better to make it not > > > > specific to device type" I thought you meant a generic --device > > > > vhost-vdpa and that's what I've been describing, but in your recent > > > > replies I guess you have a different model in mind. > > > > > > > > Are there reasons why the generic model won't work? > > > > > > I think not. > > > > > > One thing comes to my mind is that since we provide num_queues via > > > modern virtio-pci, this is probably another call for having the API > > > you described. > > > > > > For the general vhost-vdpa backend, the only thing that may block us > > > is the migration. If we want to make vhost-vdpa type independent, we > > > need first investigate the independent migration facility in virtio > > > spec which is still suspicious. > > > > Yes, definitely. > > > > Another challenge with migration is that the generic vhost-vdpa vmstate > > probably won't be compatible with QEMU's virtio-net/blk/scsi/etc > > vmstates. It would be nice if it was possible to migrate between QEMU > > and vDPA device models since they both implement the same device types. > > > > Maybe the solution is for QEMU's virtio device models to switch to the > > new VIRTIO save/load data format once that has been defined in the spec. > > Then the QEMU VirtIODevice vmstate would be: > > 1. QEMU-specific VirtIODevice state (virtqueue state, etc) > > 2. VIRTIO standard device save/load data (virtio-net mac table, etc) > > Right. The question is that do we expect the exact byte stream format > defined in the spec? It looks to me it's sufficient to define each > state that is required for the live migration and leave the byte > stream format to be implementation specific. If we manage to do this, > there's still a chance that we can live migration between those two.
Yes. I think the pros/cons of translation are better compatibility but more complex code. Not sure if maintaining a QEMU-specific save/load format in addition to the standard VIRTIO format is desirable in the long term. > > > > It's still not clear to me how much of the VIRTIO device save/load data > > is implementation-specific. I think the next step forward is to review > > the QEMU vmstates for virtio-net, virtio-gpu, etc to figure out whether > > we can really standardize the save/load data. > > Yes, and it should not be hard to have a general load and save based > on key/value pairs which could be defined in the spec. Ideally, it > should be more than enough to enumerate the keys based on the > negotiated features. (But as discussed, virtio-fs and other stateful > devices seem more complicated and a lot of spec work seems like a > requirement before support this). Great, I'm glad we had a chance to discuss this. It has helped me understand the direction things are heading in. Migration isn't a dependency for what Longpeng is doing in this patch series. The generic --device vdpa-vhost can already be implemented today without live migration support. Adding the vhost_vdpa ioctls we discussed would be nice although it seems possible (but hacky) for QEMU to probe using existing ioctls too. Longpeng: Do you want to generalize this patch into a --device vdpa-host that supports all device types? Stefan
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