Message-ID: <CACGkMEskmqv5bLyqEgXEN76Eo=NaPXd8ycMR_rs5_-PWhRkTFQ@mail.gmail.com>
Date:   Thu, 16 Dec 2021 11:43:37 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, Eli Cohen <elic@...dia.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5:
 set_features should allow reset to zero)

On Thu, Dec 16, 2021 at 4:52 AM Si-Wei Liu <si-wei.liu@...cle.com> 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@...cle.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.

That's a bug that needs to be fixed.

> 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? meaning guest not acknowledging
> VERSION_1 would use the legacy interfaces captured in the spec section
> 7.4 (regarding ring layout, native endianness, message framing, vq
> alignment of 4096, 32bit feature, no features_ok bit in status, IO port
> interface i.e. all the things) instead? Noted we don't yet have a
> set_device_features() that allows the vdpa device to tell whether it is
> operating in transitional or modern-only mode. For software virtio, all
> support for the legacy part in a transitional model has been built up
> there already, however, it's not easy for vDPA vendors to implement all
> the requirements for an all-or-nothing legacy guest support (big endian
> guest for example). To these vendors, the legacy support within a
> transitional model is more of feature to them and it's best to leave
> some flexibility for them to implement partial support for legacy. That
> in turn calls out the need for a vhost-user protocol feature like
> negotiation API that can prohibit those unsupported guest setups to as
> early as backend_init before launching the VM.
>
>
> >
> >>>> 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.
> I think the key is whether we position emulating legacy interfaces in
> QEMU doing translation on top of a v1.0 modern-only device in the
> kernel, or we allow vdpa core (or you can say vhost-vdpa) and vendor
> driver to support a transitional model in the kernel that is able to
> work for both v0.95 and v1.0 drivers, with some slight aid from QEMU for
> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for
> the former we still rely on vendor for a performant data vqs
> implementation, leaving the question to what it may end up eventually in
> the kernel is effectively the latter).

I think we can do the legacy interface emulation on top of the shadow
VQ. And we know it works for sure. But I agree, it would be much
easier if we depend on the vendor to implement a transitional device.

So assuming we depend on the vendor, I don't see anything that is
strictly needed in the kernel, the kick or config access before
DRIVER_OK can all be handled easily in Qemu unless I miss something.
The only value to do that in the kernel is that it can work for
virtio-vdpa, but modern only virito-vpda is sufficient; we don't need
any legacy stuff for that.

Thanks

>
> Thanks,
> -Siwei
>
> >
> > 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
> >>>>>>>
> >>>>>>>
>

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