Re: [PATCH] vdpa: Allow vDPA to work on big-endian machine

[Konstantin Shkolnyy <kshk@linux.ibm.com>]

On 2/12/2025 14:01, Konstantin Shkolnyy wrote:
> On 2/12/2025 12:07, Philippe Mathieu-Daudé wrote:
>> On 12/2/25 18:24, Konstantin Shkolnyy wrote:
>>> On 2/12/2025 08:52, Philippe Mathieu-Daudé wrote:
>>>> On 11/2/25 17:19, Konstantin Shkolnyy wrote:
>>>>> Add .set_vnet_le() function that always returns success, assuming that
>>>>> vDPA h/w always implements LE data format. Otherwise, QEMU disables 
>>>>> vDPA and
>>>>> outputs the message:
>>>>> "backend does not support LE vnet headers; falling back on 
>>>>> userspace virtio"
>>>>>
>>>>> Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
>>>>> ---
>>>>>   net/vhost-vdpa.c | 6 ++++++
>>>>>   1 file changed, 6 insertions(+)
>>>>>
>>>>> diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
>>>>> index 231b45246c..7219aa2eee 100644
>>>>> --- a/net/vhost-vdpa.c
>>>>> +++ b/net/vhost-vdpa.c
>>>>> @@ -270,6 +270,11 @@ static bool vhost_vdpa_has_ufo(NetClientState 
>>>>> *nc)
>>>>>   }
>>>>> +static int vhost_vdpa_set_vnet_le(NetClientState *nc, bool is_le)
>>>>> +{
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>>>   static bool vhost_vdpa_check_peer_type(NetClientState *nc, 
>>>>> ObjectClass *oc,
>>>>>                                          Error **errp)
>>>>>   {
>>>>> @@ -437,6 +442,7 @@ static NetClientInfo net_vhost_vdpa_info = {
>>>>>           .cleanup = vhost_vdpa_cleanup,
>>>>>           .has_vnet_hdr = vhost_vdpa_has_vnet_hdr,
>>>>>           .has_ufo = vhost_vdpa_has_ufo,
>>>>> +        .set_vnet_le = vhost_vdpa_set_vnet_le,
>>>>
>>>> Dubious mismatch with set_vnet_be handler.
>>>
>>> I'm not sure what you are suggesting...
>>
>> Implement set_vnet_le for parity?
> 
> To my (very limited) knowledge, kernel's vhost_vdpa that QEMU talks to 
> doesn't have an API to "change h/w endianness". If so, 
> vDPA's .set_vnet_le/be(), as well as qemu_set_vnet_le/be() have very 
> limited choices. qemu_set_vnet_le/be() behavior with vDPA was to simply 
> assume that h/w endianness by default matches host's. This assumption is 
> valid for other types of "NetClients" which are implemented in s/w. 
> However, I suspect, vDPA h/w might all be going to be LE, to match 
> virtio 1.0. Such is the NIC I'm dealing with.
> 
> My patch is only fixing a specific use case. Perhaps, for a complete 
> fix, qemu_set_vnet_be() also shouldn't unconditionally return success on 
> big endian machines, but always call .set_vnet_be() so that vDPA could 
> fail it? But then it would start calling .set_vnet_be() on other 
> "NetClients" where it didn't before.
> 
> That's why I don't want to just add a .set_vnet_be(), before someone 
> here even confirms that vDPA h/w is indeed assumed LE, and, hence, what 
> the right path is to a complete solution...
> 
> int qemu_set_vnet_be(NetClientState *nc, bool is_be)
> {
> #if HOST_BIG_ENDIAN
>      return 0;
> #else
>      if (!nc || !nc->info->set_vnet_be)
>          return -ENOSYS;
> 
>      return nc->info->set_vnet_be(nc, is_be);
> #endif
> }
> 

Does anyone have any answers/suggestions?