Re: vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero)

[Jason Wang <jasowang@redhat.com>]

On Fri, Dec 17, 2021 at 10:01 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Dec 17, 2021 at 09:57:38AM +0800, Jason Wang wrote:
> > On Fri, Dec 17, 2021 at 6:32 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
> > >
> > >
> > >
> > > On 12/15/2021 6:53 PM, Jason Wang wrote:
> > > > On Thu, Dec 16, 2021 at 10:02 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
> > > >>
> > > >>
> > > >> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:
> > > >>> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:
> > > >>>> On 12/14/2021 6:06 PM, Jason Wang wrote:
> > > >>>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
> > > >>>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote:
> > > >>>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote:
> > > >>>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote:
> > > >>>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote:
> > > >>>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion
> > > >>>>>>>>>> it ended up with. I have the following questions,
> > > >>>>>>>>>>
> > > >>>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the
> > > >>>>>>>>>> support will be completely dropped from the table, is my understanding
> > > >>>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86,
> > > >>>>>>>>>> which is backed by the spec at
> > > >>>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure
> > > >>>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier
> > > >>>>>>>>>> beyond.
> > > >>>>>>>>> I personally feel it's less work to add in kernel than try to
> > > >>>>>>>>> work around it in userspace. Jason feels differently.
> > > >>>>>>>>> Maybe post the patches and this will prove to Jason it's not
> > > >>>>>>>>> too terrible?
> > > >>>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring
> > > >>>>>>>> layout level and is limited to x86 only, there should be easy way out.
> > > >>>>>>> Note a subtle difference: what matters is that guest, not host is x86.
> > > >>>>>>> Matters for emulators which might reorder memory accesses.
> > > >>>>>>> I guess this enforcement belongs in QEMU then?
> > > >>>>>> Right, I mean to get started, the initial guest driver support and the
> > > >>>>>> corresponding QEMU support for transitional vdpa backend can be limited
> > > >>>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I
> > > >>>>>> suppose it's not hard to enforce in QEMU.
> > > >>>>> It's more than just config space, most devices have headers before the buffer.
> > > >>>> The ordering in datapath (data VQs) would have to rely on vendor's support.
> > > >>>> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays
> > > >>>> can/should well support the case when ORDER_PLATFORM is not acked by the
> > > >>>> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver
> > > >>>> today), even with v1.0 spec conforming and modern only vDPA device. The
> > > >>>> control VQ is implemented in software in the kernel, which can be easily
> > > >>>> accommodated/fixed when needed.
> > > >>>>
> > > >>>>>> QEMU can drive GET_LEGACY,
> > > >>>>>> GET_ENDIAN et al ioctls in advance to get the capability from the
> > > >>>>>> individual vendor driver. For that, we need another negotiation protocol
> > > >>>>>> similar to vhost_user's protocol_features between the vdpa kernel and
> > > >>>>>> QEMU, way before the guest driver is ever probed and its feature
> > > >>>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call
> > > >>>>>> from the device, but we can assume weak ordering for legacy at this
> > > >>>>>> point (x86 only)?
> > > >>>>> I'm lost here, we have get_features() so:
> > > >>>> I assume here you refer to get_device_features() that Eli just changed the
> > > >>>> name.
> > > >>>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie
> > > >>>>> 2) ORDER_PLATFORM means device requires platform ordering
> > > >>>>>
> > > >>>>> Any reason for having a new API for this?
> > > >>>> Are you going to enforce all vDPA hardware vendors to support the
> > > >>>> transitional model for legacy guest?
> > > > Do we really have other choices?
> > > >
> > > > I suspect the legacy device is never implemented by any vendor:
> > > >
> > > > 1) no virtio way to detect host endian
> > > This is even true for transitional device that is conforming to the
> > > spec, right?
> >
> > For hardware, yes.
> >
> > > The transport specific way to detect host endian is still
> > > being discussed and the spec revision is not finalized yet so far as I
> > > see. Why this suddenly becomes a requirement/blocker for h/w vendors to
> > > implement the transitional model?
> >
> > It's not a sudden blocker, the problem has existed since day 0 if I
> > was not wrong. That's why the problem looks a little bit complicated
> > and why it would be much simpler if we stick to modern devices.
> >
> > > Even if the spec is out, this is
> > > pretty new and I suspect not all vendor would follow right away. I hope
> > > the software framework can be tolerant with h/w vendors not supporting
> > > host endianess (BE specifically) or not detecting it if they would like
> > > to support a transitional device for legacy.
> >
> > Well, if we know we don't want to support the BE host it would be fine.
>
> I think you guys mean guest not host here. Same for memory ordering etc.
> What matters is whether guest has barriers etc.
>

Yes.

Thanks