Re: [PATCH net-next v9 3/6] tun: Introduce virtio-net hash feature

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2025/03/10 13:01, Jason Wang wrote:
> On Mon, Mar 10, 2025 at 11:55 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> On Fri, Mar 7, 2025 at 7:01 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>>
>>> Hash reporting
>>> ==============
>>>
>>> Allow the guest to reuse the hash value to make receive steering
>>> consistent between the host and guest, and to save hash computation.
>>>
>>> RSS
>>> ===
>>>
>>> RSS is a receive steering algorithm that can be negotiated to use with
>>> virtio_net. Conventionally the hash calculation was done by the VMM.
>>> However, computing the hash after the queue was chosen defeats the
>>> purpose of RSS.
>>>
>>> Another approach is to use eBPF steering program. This approach has
>>> another downside: it cannot report the calculated hash due to the
>>> restrictive nature of eBPF steering program.
>>>
>>> Introduce the code to perform RSS to the kernel in order to overcome
>>> thse challenges. An alternative solution is to extend the eBPF steering
>>> program so that it will be able to report to the userspace, but I didn't
>>> opt for it because extending the current mechanism of eBPF steering
>>> program as is because it relies on legacy context rewriting, and
>>> introducing kfunc-based eBPF will result in non-UAPI dependency while
>>> the other relevant virtualization APIs such as KVM and vhost_net are
>>> UAPIs.
>>>
>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>>> Tested-by: Lei Yang <leiyang@redhat.com>
>>> ---
>>>   Documentation/networking/tuntap.rst |   7 ++
>>>   drivers/net/Kconfig                 |   1 +
>>>   drivers/net/tap.c                   |  68 ++++++++++++++-
>>>   drivers/net/tun.c                   |  98 +++++++++++++++++-----
>>>   drivers/net/tun_vnet.h              | 159 ++++++++++++++++++++++++++++++++++--
>>>   include/linux/if_tap.h              |   2 +
>>>   include/linux/skbuff.h              |   3 +
>>>   include/uapi/linux/if_tun.h         |  75 +++++++++++++++++
>>>   net/core/skbuff.c                   |   4 +
>>>   9 files changed, 386 insertions(+), 31 deletions(-)
> 
> [...]
> 
>>> + *
>>> + * The %TUN_VNET_HASH_REPORT flag set with this ioctl will be effective only
>>> + * after calling the %TUNSETVNETHDRSZ ioctl with a number greater than or equal
>>> + * to the size of &struct virtio_net_hdr_v1_hash.
>>
>> So you had a dependency check already for vnet hdr len. I'd still
>> suggest to split this into rss and hash as they are separated
>> features. Then we can use separate data structure for them instead of
>> a container struct.
>>
> 
> Besides this, I think we still need to add new bits to TUNGETIFF to
> let userspace know about the new ability.

The userspace can peform TUNGETVNETHASHCAP and see if it results in EINVAL.

Regards,
Akihiko Odaki

> 
> Thanks
>