Message-ID: <CACGkMEszKB3Cdk4H-1De5=ZQ=fp47vPntS-ww9_V0tC26d8bKw@mail.gmail.com>
Date:   Fri, 17 Dec 2021 10:01:23 +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 Fri, Dec 17, 2021 at 9:08 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
>
>
>
> On 12/15/2021 7:43 PM, Jason Wang wrote:
> > 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.
> First I am not sure if there's a convincing case for users to deploy
> vDPA with shadow (data) VQ against the pure software based backend.
> Please enlighten me if there is.

The problem is shadow VQ is the only solution that can works for all the cases.

>
> For us, the point to deploy vDPA for legacy guest is the acceleration
> (what "A" stands for in "vDPA") part of it so that we can leverage the
> hardware potential if at all possible. Not sure how the shadow VQ
> implementation can easily deal with datapath acceleration without losing
> too much performance?

It's not easy, shadow VQ will lose performance for sure.

>
> > 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.
> Right, that's what I think too it's not quite a lot of work in the
> kernel if vendor device offers the aid/support for transitional. The
> kernel only provides the abstraction of device model (transitional or
> modern-only), while vendor driver may implement early platform feature
> discovery and apply legacy specific quirks (unsupported endianness,
> mismatched page size, unsupported host memory ordering model) that the
> device can't adapt to. I don't say we have to depend on the vendor, but
> the point is that we must assume fully spec compliant transitional
> support (the datapath in particular) from the vendor to get started, as
> I guess it's probably the main motivation for users to deploy it -
> acceleration of legacy guest workload without exhausting host computing
> resource. Even if we get started with shadow VQ to mediate and translate
> the datapath, eventually it may evolve towards leveraging datapath
> offload to hardware if acceleration is the only convincing use case for
> legacy support.

Yes, so as discussed, I don't object the idea, kernel patches are more
than welcomed.

Thanks

>
> Thanks,
> -Siwei
> > 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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