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    SubjectRe: vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero)
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    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? 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.
    > Right. Of note is the fact that it's a spec bug which I
    > hope yet to fix, though due to existing guest code the
    > fix won't be complete.
    I thought at one point you pointed out to me that the spec does allow
    config space read before claiming features_ok, and only config write
    before features_ok is prohibited. I haven't read up the full thread of
    Halil's VERSION_1 for transitional big endian device yet, but what is
    the spec bug you hope to fix?

    >
    > WRT ioctls, One thing we can do though is abuse set_features
    > where it's called by QEMU early on with just the VERSION_1
    > bit set, to distinguish between legacy and modern
    > interface. This before config space accesses and FEATURES_OK.
    >
    > Halil has been working on this, pls take a look and maybe help him out.
    Interesting thread, am reading now and see how I may leverage or help there.

    >>>>>> 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).
    >>
    >> Thanks,
    >> -Siwei
    >
    > My suggestion is post the kernel patches, and we can evaluate
    > how much work they are.
    Thanks for the feedback. I will take some read then get back, probably
    after the winter break. Stay tuned.

    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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