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

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Message-ID: <39c059c9-fe67-46e4-8c81-854a3de8d726@daynix.com>
Date: Wed, 12 Mar 2025 14:55:36 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Jonathan Corbet <corbet@....net>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
 Yuri Benditovich <yuri.benditovich@...nix.com>,
 Andrew Melnychenko <andrew@...nix.com>,
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com,
 Lei Yang <leiyang@...hat.com>, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next v9 3/6] tun: Introduce virtio-net hash feature

On 2025/03/12 11:59, Jason Wang wrote:
> On Tue, Mar 11, 2025 at 2:17 PM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>
>> On 2025/03/11 9:38, Jason Wang wrote:
>>> On Mon, Mar 10, 2025 at 3:45 PM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>>>
>>>> On 2025/03/10 12:55, Jason Wang wrote:
>>>>> On Fri, Mar 7, 2025 at 7:01 PM Akihiko Odaki <akihiko.odaki@...nix.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@...nix.com>
>>>>>> Tested-by: Lei Yang <leiyang@...hat.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(-)
>>>>>>
> 
> [...]
> 
>>>>> Let's has a consistent name for this and the uapi to be consistent
>>>>> with TUNSETIFF/TUNGETIFF. Probably TUNSETVNETHASH and
>>>>> tun_vnet_ioctl_gethash().
>>>>
>>>> They have different semantics so they should have different names.
>>>> TUNGETIFF reports the value currently set while TUNGETVNETHASHCAP
>>>> reports the value that can be set later.
>>>
>>> I'm not sure I will get here. I meant a symmetric name
>>>
>>> TUNSETVNETHASH and TUNVETVNETHASH.
>>
>> TUNGETVNETHASHCAP does not correspond to TUNGETIFF. The correspondence
>> of ioctl names is as follows:
>> TUNGETFEATURES - TUNGETVNETHASHCAP
> 
> TUNGETFEATURES returns the value set from TUNSETIFF. This differs from
> TUNGETVNETHASHCAP semantic which just return the capabilities.
> 
> +static inline long tun_vnet_ioctl_gethashcap(void __user *argp)
> +{
> +       static const struct tun_vnet_hash cap = {
> +               .flags = TUN_VNET_HASH_REPORT | TUN_VNET_HASH_RSS,
> +               .types = VIRTIO_NET_SUPPORTED_HASH_TYPES
> +       };
> +
> +       return copy_to_user(argp, &cap, sizeof(cap)) ? -EFAULT : 0;
> +}
> 
> TUNGETFEATURES doesn't' help too much for non-persist TAP as userspace
> knows what value it set before.
> 
>> TUNSETIFF - TUNSETVNETHASH
>> TUNGETIFF - no corresponding ioctl for the virtio-net hash features
> 
> And this sounds odd and a hint for a incomplete uAPI as userspace
> needs to know knowing what can set before doing TUNSETVNETHASH.

You are confused with TUNGETFEATURES and TUNGETIFF. Below is the code 
that implements TUNGETFEATURES:
if (cmd == TUNGETFEATURES) {
	/* Currently this just means: "what IFF flags are valid?".
	 * This is needed because we never checked for invalid flags on
	 * TUNSETIFF.
	 */
	return put_user(IFF_TUN | IFF_TAP | IFF_NO_CARRIER |
			TUN_FEATURES, (unsigned int __user*)argp);
} else if (cmd == TUNSETQUEUE) {

Regards,
Akihiko Odaki

> 
>>
>> Regards,
>> Akihiko Odaki
>>
> 
> Thanks
>