On Tue, Mar 11, 2025 at 2:17 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
>
> On 2025/03/11 9:38, Jason Wang wrote:
> > On Mon, Mar 10, 2025 at 3:45 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>
> >> On 2025/03/10 12:55, Jason Wang wrote:
> >>> On Fri, Mar 7, 2025 at 7:01 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> 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@xxxxxxxxxx>
> >>>> Tested-by: Lei Yang <leiyang@xxxxxxxxxx>
> >>>> ---
> >>>> 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.
>
> Regards,
> Akihiko Odaki
>
Thanks
