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

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

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
}