Message-ID: <CACGkMEv24Rn9+bJ5mma1ciJNwa7wvRCwJ6jF+CBMbz6DtV8MvA@mail.gmail.com>
Date:   Tue, 23 Aug 2022 11:26:11 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>
Cc:     Si-Wei Liu <si-wei.liu@...cle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Parav Pandit <parav@...dia.com>,
        Yongji Xie <xieyongji@...edance.com>,
        "Dawar, Gautam" <gautam.dawar@....com>
Subject: Re: [PATCH 2/2] vDPA: conditionally read fields in virtio-net dev

On Mon, Aug 22, 2022 at 1:08 PM Zhu, Lingshan <lingshan.zhu@...el.com> wrote:
>
>
>
> On 8/20/2022 4:55 PM, Si-Wei Liu wrote:
> >
> >
> > On 8/18/2022 5:42 PM, Jason Wang wrote:
> >> On Fri, Aug 19, 2022 at 7:20 AM Si-Wei Liu <si-wei.liu@...cle.com>
> >> wrote:
> >>>
> >>>
> >>> On 8/17/2022 9:15 PM, Jason Wang wrote:
> >>>> 在 2022/8/17 18:37, Michael S. Tsirkin 写道:
> >>>>> On Wed, Aug 17, 2022 at 05:43:22PM +0800, Zhu, Lingshan wrote:
> >>>>>> On 8/17/2022 5:39 PM, Michael S. Tsirkin wrote:
> >>>>>>> On Wed, Aug 17, 2022 at 05:13:59PM +0800, Zhu, Lingshan wrote:
> >>>>>>>> On 8/17/2022 4:55 PM, Michael S. Tsirkin wrote:
> >>>>>>>>> On Wed, Aug 17, 2022 at 10:14:26AM +0800, Zhu, Lingshan wrote:
> >>>>>>>>>> Yes it is a little messy, and we can not check _F_VERSION_1
> >>>>>>>>>> because of
> >>>>>>>>>> transitional devices, so maybe this is the best we can do for
> >>>>>>>>>> now
> >>>>>>>>> I think vhost generally needs an API to declare config space
> >>>>>>>>> endian-ness
> >>>>>>>>> to kernel. vdpa can reuse that too then.
> >>>>>>>> Yes, I remember you have mentioned some IOCTL to set the
> >>>>>>>> endian-ness,
> >>>>>>>> for vDPA, I think only the vendor driver knows the endian,
> >>>>>>>> so we may need a new function vdpa_ops->get_endian().
> >>>>>>>> In the last thread, we say maybe it's better to add a comment for
> >>>>>>>> now.
> >>>>>>>> But if you think we should add a vdpa_ops->get_endian(), I can
> >>>>>>>> work
> >>>>>>>> on it for sure!
> >>>>>>>>
> >>>>>>>> Thanks
> >>>>>>>> Zhu Lingshan
> >>>>>>> I think QEMU has to set endian-ness. No one else knows.
> >>>>>> Yes, for SW based vhost it is true. But for HW vDPA, only
> >>>>>> the device & driver knows the endian, I think we can not
> >>>>>> "set" a hardware's endian.
> >>>>> QEMU knows the guest endian-ness and it knows that
> >>>>> device is accessed through the legacy interface.
> >>>>> It can accordingly send endian-ness to the kernel and
> >>>>> kernel can propagate it to the driver.
> >>>>
> >>>> I wonder if we can simply force LE and then Qemu can do the endian
> >>>> conversion?
> >>> convert from LE for config space fields only, or QEMU has to forcefully
> >>> mediate and covert endianness for all device memory access including
> >>> even the datapath (fields in descriptor and avail/used rings)?
> >> Former. Actually, I want to force modern devices for vDPA when
> >> developing the vDPA framework. But then we see requirements for
> >> transitional or even legacy (e.g the Ali ENI parent). So it
> >> complicates things a lot.
> >>
> >> I think several ideas has been proposed:
> >>
> >> 1) Your proposal of having a vDPA specific way for
> >> modern/transitional/legacy awareness. This seems very clean since each
> >> transport should have the ability to do that but it still requires
> >> some kind of mediation for the case e.g running BE legacy guest on LE
> >> host.
> > In theory it seems like so, though practically I wonder if we can just
> > forbid BE legacy driver from running on modern LE host. For those who
> > care about legacy BE guest, they mostly like could and should talk to
> > vendor to get native BE support to achieve hardware acceleration,

The problem is the hardware still needs a way to know the endian of the guest?

> > few
> > of them would count on QEMU in mediating or emulating the datapath
> > (otherwise I don't see the benefit of adopting vDPA?). I still feel
> > that not every hardware vendor has to offer backward compatibility
> > (transitional device) with legacy interface/behavior (BE being just
> > one),

Probably, I agree it is a corner case, and dealing with transitional
device for the following setups is very challenge for hardware:

- driver without IOMMU_PLATFORM support, (requiring device to send
translated request which have security implications)
- BE legacy guest on LE host, (requiring device to have a way to know
the endian)
- device specific requirement (e.g modern virtio-net mandate minimal
header length to contain mrg_rxbuf even if the device doesn't offer
it)

It is not obvious for the hardware vendor, so we may end up defecting
in the implementation. Dealing with compatibility for the transitional
devices is kind of a nightmare which there's no way for the spec to
rule the behavior of legacy devices.

> >  this is unlike the situation on software virtio device, which
> > has legacy support since day one. I think we ever discussed it before:
> > for those vDPA vendors who don't offer legacy guest support, maybe we
> > should mandate some feature for e.g. VERSION_1, as these devices
> > really don't offer functionality of the opposite side (!VERSION_1)
> > during negotiation.

I've tried something similar here (a global mandatory instead of per device).

https://lkml.org/lkml/2021/6/4/26

But for some reason, it is not applied by Michael. It would be a great
relief if we support modern devices only. Maybe it's time to revisit
this idea then we can introduce new backend features and then we can
mandate VERSION_1

> >
> > Having it said, perhaps we should also allow vendor device to
> > implement only partial support for legacy. We can define "reversed"
> > backend feature to denote some part of the legacy
> > interface/functionality not getting implemented by device. For
> > instance, VHOST_BACKEND_F_NO_BE_VRING, VHOST_BACKEND_F_NO_BE_CONFIG,
> > VHOST_BACKEND_F_NO_ALIGNED_VRING,
> > VHOST_BACKEND_NET_F_NO_WRITEABLE_MAC, and et al. Not all of these
> > missing features for legacy would be easy for QEMU to make up for, so
> > QEMU can selectively emulate those at its best when necessary and
> > applicable. In other word, this design shouldn't prevent QEMU from
> > making up for vendor device's partial legacy support.

This looks too heavyweight since it tries to provide compatibility for
legacy drivers. Considering we've introduced modern devices for 5+
years, I'd rather:

- Qemu to mediate the config space stuffs
- Shadow virtqueue to mediate the datapath (AF_XDP told us shadow ring
can perform very well if we do zero-copy).

> >
> >>
> >> 2) Michael suggests using VHOST_SET_VRING_ENDIAN where it means we
> >> need a new config ops for vDPA bus, but it doesn't solve the issue for
> >> config space (at least from its name). We probably need a new ioctl
> >> for both vring and config space.
> > Yep adding a new ioctl makes things better, but I think the key is not
> > the new ioctl. It's whether or not we should enforce every vDPA vendor
> > driver to implement all transitional interfaces to be spec compliant.

I think the answer is no since the spec allows transitional device.
And we know things will be greatly simplified if vDPA support non
transitional device only.

So we can change the question to:

1) do we need (or is it too late) to enforce non transitional device?
2) if yes, can transitional device be mediate in an efficient way?

For 1), it's probably too late but we can invent new vDPA features as
you suggest to be non transitional. Then we can:

1.1) extend the netlink API to provision non-transitonal device
1.2) work on the non-transtional device in the future
1.3) presenting transitional device via mediation

The previous transitional vDPA work as is, it's probably too late to
fix all the issue we suffer.

For 2), the key part is the datapath mediation, we can use shadow virtqueue.

> > If we allow them to reject the VHOST_SET_VRING_ENDIAN  or
> > VHOST_SET_CONFIG_ENDIAN call, what could we do? We would still end up
> > with same situation of either fail the guest, or trying to
> > mediate/emulate, right?
> >
> > Not to mention VHOST_SET_VRING_ENDIAN is rarely supported by vhost
> > today - few distro kernel has CONFIG_VHOST_CROSS_ENDIAN_LEGACY enabled
> > and QEMU just ignores the result. vhost doesn't necessarily depend on
> > it to determine endianness it looks.
> I would like to suggest to add two new config ops get/set_vq_endian()
> and get/set_config_endian() for vDPA. This is used to:
> a) support VHOST_GET/SET_VRING_ENDIAN as MST suggested, and add
> VHOST_SET/GET_CONFIG_ENDIAN for vhost_vdpa.
> If the device has not implemented interface to set its endianess, then
> no matter success or failure of SET_ENDIAN, QEMU knows the endian-ness
> anyway.

How can Qemu know the endian in this way? And if it can, there's no
need for the new API?

> In this case, if the device endianess does not match the guest,
> there needs a mediation layer or fail.
> b) ops->get_config_endian() can always tell the endian-ness of the
> device config space after the vendor driver probing the device. So we
> can use this ops->get_config_endian() for
> MTU, MAC and other fields handling in vdpa_dev_net_config_fill() and we
> don't need to set_features in vdpa_get_config_unlocked(), so no race
> conditions.
> Every time ops->get_config() returned, we can tell the endian by
> ops-config_>get_endian(), we don't need set_features(xxx, 0) if features
> negotiation not done.
>
> The question is: Do we need two pairs of ioctls for both vq and config
> space? Can config space endian-ness differ from the vqs?
> c) do we need a new netlink attr telling the endian-ness to user space?

Generally, I'm not sure this is a good design consider it provides neither:

Compatibility with the virtio spec

nor

Compatibility with the existing vhost API (VHOST_SET_VRING_ENDIAN)

Thanks

>
> Thanks,
> Zhu Lingshan
> >
> >>
> >> or
> >>
> >> 3) revisit the idea of forcing modern only device which may simplify
> >> things a lot
> > I am not actually against forcing modern only config space, given that
> > it's not hard for either QEMU or individual driver to mediate or
> > emulate, and for the most part it's not conflict with the goal of
> > offload or acceleration with vDPA. But forcing LE ring layout IMO
> > would just kill off the potential of a very good use case. Currently
> > for our use case the priority for supporting 0.9.5 guest with vDPA is
> > slightly lower compared to live migration, but it is still in our TODO
> > list.
> >
> > Thanks,
> > -Siwei
> >
> >>
> >> which way should we go?
> >>
> >>> I hope
> >>> it's not the latter, otherwise it loses the point to use vDPA for
> >>> datapath acceleration.
> >>>
> >>> Even if its the former, it's a little weird for vendor device to
> >>> implement a LE config space with BE ring layout, although still
> >>> possible...
> >> Right.
> >>
> >> Thanks
> >>
> >>> -Siwei
> >>>> Thanks
> >>>>
> >>>>
> >>>>>> So if you think we should add a vdpa_ops->get_endian(),
> >>>>>> I will drop these comments in the next version of
> >>>>>> series, and work on a new patch for get_endian().
> >>>>>>
> >>>>>> Thanks,
> >>>>>> Zhu Lingshan
> >>>>> Guests don't get endian-ness from devices so this seems pointless.
> >>>>>
> >
>

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