Re: [PATCH 2/2] vDPA: conditionally read fields in virtio-net dev

On 8/20/2022 4:55 PM, Si-Wei Liu wrote:

On 8/18/2022 5:42 PM, Jason Wang wrote:

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On Fri, Aug 19, 2022 at 7:20 AM Si-Wei Liu [off-list ref] 
wrote:

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On 8/17/2022 9:15 PM, Jason Wang wrote:

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在 2022/8/17 18:37, Michael S. Tsirkin 写道:

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On Wed, Aug 17, 2022 at 05:43:22PM +0800, Zhu, Lingshan wrote:

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On 8/17/2022 5:39 PM, Michael S. Tsirkin wrote:

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On Wed, Aug 17, 2022 at 05:13:59PM +0800, Zhu, Lingshan wrote:

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On 8/17/2022 4:55 PM, Michael S. Tsirkin wrote:

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On Wed, Aug 17, 2022 at 10:14:26AM +0800, Zhu, Lingshan wrote:

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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, 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), 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.

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.

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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. 
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. 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?

Thanks,
Zhu Lingshan

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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

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which way should we go?

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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

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-Siwei

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Thanks

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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.