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

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Message-ID: <4f11f53f-c6d4-c7eb-de7b-0260942464fe@intel.com>
Date:   Thu, 18 Aug 2022 15:58:13 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Si-Wei Liu <si-wei.liu@...cle.com>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, parav@...dia.com, xieyongji@...edance.com,
        gautam.dawar@....com
Subject: 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:
>> 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?
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
>
>
>>
>>> 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.
>>
>