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

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Date:   Thu, 18 Aug 2022 12:15:37 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        "Zhu, Lingshan" <lingshan.zhu@...el.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


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?

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.
>