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

On 8/18/2022 12:15 PM, Jason Wang wrote:

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?

I think this is what we are doing now, force it to be LE, leave a comment.

QEMU will not set ENDIAN for vDPA devices, vhost_kernel_call() verifies
whether the backend is TYPE_KERNEL (we have TYPE_VDPA here),
so we can not rely on this code path.

Thanks
Zhu Lingshan

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.