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

[Jason Wang <jasowang@redhat.com>]

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.

> 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:

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?

>
> >
> >> I
> >> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level
> >> 0.95 support, it seems all the ingredient had been there already dated back
> >> to the DPDK days. The only major thing limiting is in the vDPA software that
> >> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for
> >> a few DMA setup ops, which is virtio 1.0 only.
> >>
> >>>> 2. suppose some form of legacy guest support needs to be there, how do we
> >>>> deal with the bogus assumption below in vdpa_get_config() in the short term?
> >>>> It looks one of the intuitive fix is to move the vdpa_set_features call out
> >>>> of vdpa_get_config() to vdpa_set_config().
> >>>>
> >>>>           /*
> >>>>            * Config accesses aren't supposed to trigger before features are
> >>>> set.
> >>>>            * If it does happen we assume a legacy guest.
> >>>>            */
> >>>>           if (!vdev->features_valid)
> >>>>                   vdpa_set_features(vdev, 0);
> >>>>           ops->get_config(vdev, offset, buf, len);
> >>>>
> >>>> I can post a patch to fix 2) if there's consensus already reached.
> >>>>
> >>>> Thanks,
> >>>> -Siwei
> >>> I'm not sure how important it is to change that.
> >>> In any case it only affects transitional devices, right?
> >>> Legacy only should not care ...
> >> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the
> >> modern one in a transitional device model rather than being legacy only.
> >> That way a v0.95 and v1.0 supporting vdpa parent can support both types of
> >> guests without having to reconfigure. Or are you suggesting limit to legacy
> >> only at the time of vdpa creation would simplify the implementation a lot?
> >>
> >> Thanks,
> >> -Siwei
> >
> > I don't know for sure. Take a look at the work Halil was doing
> > to try and support transitional devices with BE guests.
> Hmmm, we can have those endianness ioctls defined but the initial QEMU
> implementation can be started to support x86 guest/host with little
> endian and weak memory ordering first. The real trick is to detect
> legacy guest - I am not sure if it's feasible to shift all the legacy
> detection work to QEMU, or the kernel has to be part of the detection
> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking
> effort in QEMU) as well. Let me take a further look and get back.

Michael may think differently but I think doing this in Qemu is much easier.

Thanks



>
> Meanwhile, I'll check internally to see if a legacy only model would
> work. Thanks.
>
> Thanks,
> -Siwei
>
>
> >
> >
> >>>> On 3/2/2021 2:53 AM, Jason Wang wrote:
> >>>>> On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote:
> >>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote:
> >>>>>>> On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote:
> >>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> >>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify
> >>>>>>>>>> the kernel that it's a legacy guest. Ugh :(
> >>>>>>>>> Well, although I think adding an ioctl is doable, may I
> >>>>>>>>> know what the use
> >>>>>>>>> case there will be for kernel to leverage such info
> >>>>>>>>> directly? Is there a
> >>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed
> >>>>>>>>> differentiation (legacy v.s. modern) needed?
> >>>>>>>> BTW a good API could be
> >>>>>>>>
> >>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int)
> >>>>>>>>
> >>>>>>>> we did it per vring but maybe that was a mistake ...
> >>>>>>> Actually, I wonder whether it's good time to just not support
> >>>>>>> legacy driver
> >>>>>>> for vDPA. Consider:
> >>>>>>>
> >>>>>>> 1) It's definition is no-normative
> >>>>>>> 2) A lot of budren of codes
> >>>>>>>
> >>>>>>> So qemu can still present the legacy device since the config
> >>>>>>> space or other
> >>>>>>> stuffs that is presented by vhost-vDPA is not expected to be
> >>>>>>> accessed by
> >>>>>>> guest directly. Qemu can do the endian conversion when necessary
> >>>>>>> in this
> >>>>>>> case?
> >>>>>>>
> >>>>>>> Thanks
> >>>>>>>
> >>>>>> Overall I would be fine with this approach but we need to avoid breaking
> >>>>>> working userspace, qemu releases with vdpa support are out there and
> >>>>>> seem to work for people. Any changes need to take that into account
> >>>>>> and document compatibility concerns.
> >>>>> Agree, let me check.
> >>>>>
> >>>>>
> >>>>>>     I note that any hardware
> >>>>>> implementation is already broken for legacy except on platforms with
> >>>>>> strong ordering which might be helpful in reducing the scope.
> >>>>> Yes.
> >>>>>
> >>>>> Thanks
> >>>>>
> >>>>>
>