Re: [PATCH net-next v12 01/10] virtio_net: Add functions for hashing

[Jason Wang <jasowang@redhat.com>]

On Thu, Jun 5, 2025 at 3:58 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> On 2025/06/05 10:53, Jason Wang wrote:
> > On Wed, Jun 4, 2025 at 3:20 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> >>
> >> On 2025/06/04 10:18, Jason Wang wrote:
> >>> On Tue, Jun 3, 2025 at 1:31 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> >>>>
> >>>> On 2025/06/03 12:19, Jason Wang wrote:
> >>>>> On Fri, May 30, 2025 at 12:50 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> >>>>>>
> >>>>>> They are useful to implement VIRTIO_NET_F_RSS and
> >>>>>> VIRTIO_NET_F_HASH_REPORT.
> >>>>>>
> >>>>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> >>>>>> Tested-by: Lei Yang <leiyang@redhat.com>
> >>>>>> ---
> >>>>>>     include/linux/virtio_net.h | 188 +++++++++++++++++++++++++++++++++++++++++++++
> >>>>>>     1 file changed, 188 insertions(+)
> >>>>>>
> >>>>>> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> >>>>>> index 02a9f4dc594d..426f33b4b824 100644
> >>>>>> --- a/include/linux/virtio_net.h
> >>>>>> +++ b/include/linux/virtio_net.h
> >>>>>> @@ -9,6 +9,194 @@
> >>>>>>     #include <uapi/linux/tcp.h>
> >>>>>>     #include <uapi/linux/virtio_net.h>
> >>>>>>
> >>>>>> +struct virtio_net_hash {
> >>>>>> +       u32 value;
> >>>>>> +       u16 report;
> >>>>>> +};
> >>>>>> +
> >>>>>> +struct virtio_net_toeplitz_state {
> >>>>>> +       u32 hash;
> >>>>>> +       const u32 *key;
> >>>>>> +};
> >>>>>> +
> >>>>>> +#define VIRTIO_NET_SUPPORTED_HASH_TYPES (VIRTIO_NET_RSS_HASH_TYPE_IPv4 | \
> >>>>>> +                                        VIRTIO_NET_RSS_HASH_TYPE_TCPv4 | \
> >>>>>> +                                        VIRTIO_NET_RSS_HASH_TYPE_UDPv4 | \
> >>>>>> +                                        VIRTIO_NET_RSS_HASH_TYPE_IPv6 | \
> >>>>>> +                                        VIRTIO_NET_RSS_HASH_TYPE_TCPv6 | \
> >>>>>> +                                        VIRTIO_NET_RSS_HASH_TYPE_UDPv6)
> >>>>>> +
> >>>>>> +#define VIRTIO_NET_RSS_MAX_KEY_SIZE 40
> >>>>>> +
> >>>>>> +static inline void virtio_net_toeplitz_convert_key(u32 *input, size_t len)
> >>>>>> +{
> >>>>>> +       while (len >= sizeof(*input)) {
> >>>>>> +               *input = be32_to_cpu((__force __be32)*input);
> >>>>>> +               input++;
> >>>>>> +               len -= sizeof(*input);
> >>>>>> +       }
> >>>>>> +}
> >>>>>> +
> >>>>>> +static inline void virtio_net_toeplitz_calc(struct virtio_net_toeplitz_state *state,
> >>>>>> +                                           const __be32 *input, size_t len)
> >>>>>> +{
> >>>>>> +       while (len >= sizeof(*input)) {
> >>>>>> +               for (u32 map = be32_to_cpu(*input); map; map &= (map - 1)) {
> >>>>>> +                       u32 i = ffs(map);
> >>>>>> +
> >>>>>> +                       state->hash ^= state->key[0] << (32 - i) |
> >>>>>> +                                      (u32)((u64)state->key[1] >> i);
> >>>>>> +               }
> >>>>>> +
> >>>>>> +               state->key++;
> >>>>>> +               input++;
> >>>>>> +               len -= sizeof(*input);
> >>>>>> +       }
> >>>>>> +}
> >>>>>> +
> >>>>>> +static inline u8 virtio_net_hash_key_length(u32 types)
> >>>>>> +{
> >>>>>> +       size_t len = 0;
> >>>>>> +
> >>>>>> +       if (types & VIRTIO_NET_HASH_REPORT_IPv4)
> >>>>>> +               len = max(len,
> >>>>>> +                         sizeof(struct flow_dissector_key_ipv4_addrs));
> >>>>>> +
> >>>>>> +       if (types &
> >>>>>> +           (VIRTIO_NET_HASH_REPORT_TCPv4 | VIRTIO_NET_HASH_REPORT_UDPv4))
> >>>>>> +               len = max(len,
> >>>>>> +                         sizeof(struct flow_dissector_key_ipv4_addrs) +
> >>>>>> +                         sizeof(struct flow_dissector_key_ports));
> >>>>>> +
> >>>>>> +       if (types & VIRTIO_NET_HASH_REPORT_IPv6)
> >>>>>> +               len = max(len,
> >>>>>> +                         sizeof(struct flow_dissector_key_ipv6_addrs));
> >>>>>> +
> >>>>>> +       if (types &
> >>>>>> +           (VIRTIO_NET_HASH_REPORT_TCPv6 | VIRTIO_NET_HASH_REPORT_UDPv6))
> >>>>>> +               len = max(len,
> >>>>>> +                         sizeof(struct flow_dissector_key_ipv6_addrs) +
> >>>>>> +                         sizeof(struct flow_dissector_key_ports));
> >>>>>> +
> >>>>>> +       return len + sizeof(u32);
> >>>>>> +}
> >>>>>> +
> >>>>>> +static inline u32 virtio_net_hash_report(u32 types,
> >>>>>> +                                        const struct flow_keys_basic *keys)
> >>>>>> +{
> >>>>>> +       switch (keys->basic.n_proto) {
> >>>>>> +       case cpu_to_be16(ETH_P_IP):
> >>>>>> +               if (!(keys->control.flags & FLOW_DIS_IS_FRAGMENT)) {
> >>>>>> +                       if (keys->basic.ip_proto == IPPROTO_TCP &&
> >>>>>> +                           (types & VIRTIO_NET_RSS_HASH_TYPE_TCPv4))
> >>>>>> +                               return VIRTIO_NET_HASH_REPORT_TCPv4;
> >>>>>> +
> >>>>>> +                       if (keys->basic.ip_proto == IPPROTO_UDP &&
> >>>>>> +                           (types & VIRTIO_NET_RSS_HASH_TYPE_UDPv4))
> >>>>>> +                               return VIRTIO_NET_HASH_REPORT_UDPv4;
> >>>>>> +               }
> >>>>>> +
> >>>>>> +               if (types & VIRTIO_NET_RSS_HASH_TYPE_IPv4)
> >>>>>> +                       return VIRTIO_NET_HASH_REPORT_IPv4;
> >>>>>> +
> >>>>>> +               return VIRTIO_NET_HASH_REPORT_NONE;
> >>>>>> +
> >>>>>> +       case cpu_to_be16(ETH_P_IPV6):
> >>>>>> +               if (!(keys->control.flags & FLOW_DIS_IS_FRAGMENT)) {
> >>>>>> +                       if (keys->basic.ip_proto == IPPROTO_TCP &&
> >>>>>> +                           (types & VIRTIO_NET_RSS_HASH_TYPE_TCPv6))
> >>>>>> +                               return VIRTIO_NET_HASH_REPORT_TCPv6;
> >>>>>> +
> >>>>>> +                       if (keys->basic.ip_proto == IPPROTO_UDP &&
> >>>>>> +                           (types & VIRTIO_NET_RSS_HASH_TYPE_UDPv6))
> >>>>>> +                               return VIRTIO_NET_HASH_REPORT_UDPv6;
> >>>>>> +               }
> >>>>>> +
> >>>>>> +               if (types & VIRTIO_NET_RSS_HASH_TYPE_IPv6)
> >>>>>> +                       return VIRTIO_NET_HASH_REPORT_IPv6;
> >>>>>> +
> >>>>>> +               return VIRTIO_NET_HASH_REPORT_NONE;
> >>>>>> +
> >>>>>> +       default:
> >>>>>> +               return VIRTIO_NET_HASH_REPORT_NONE;
> >>>>>> +       }
> >>>>>> +}
> >>>>>> +
> >>>>>> +static inline void virtio_net_hash_rss(const struct sk_buff *skb,
> >>>>>> +                                      u32 types, const u32 *key,
> >>>>>> +                                      struct virtio_net_hash *hash)
> >>>>>> +{
> >>>>>> +       struct virtio_net_toeplitz_state toeplitz_state = { .key = key };
> >>>>>> +       struct flow_keys flow;
> >>>>>> +       struct flow_keys_basic flow_basic;
> >>>>>> +       u16 report;
> >>>>>> +
> >>>>>> +       if (!skb_flow_dissect_flow_keys(skb, &flow, 0)) {
> >>>>>> +               hash->report = VIRTIO_NET_HASH_REPORT_NONE;
> >>>>>> +               return;
> >>>>>> +       }
> >>>>>> +
> >>>>>> +       flow_basic = (struct flow_keys_basic) {
> >>>>>> +               .control = flow.control,
> >>>>>> +               .basic = flow.basic
> >>>>>> +       };
> >>>>>> +
> >>>>>> +       report = virtio_net_hash_report(types, &flow_basic);
> >>>>>> +
> >>>>>> +       switch (report) {
> >>>>>> +       case VIRTIO_NET_HASH_REPORT_IPv4:
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state,
> >>>>>> +                                        (__be32 *)&flow.addrs.v4addrs,
> >>>>>> +                                        sizeof(flow.addrs.v4addrs));
> >>>>>> +               break;
> >>>>>> +
> >>>>>> +       case VIRTIO_NET_HASH_REPORT_TCPv4:
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state,
> >>>>>> +                                        (__be32 *)&flow.addrs.v4addrs,
> >>>>>> +                                        sizeof(flow.addrs.v4addrs));
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state, &flow.ports.ports,
> >>>>>> +                                        sizeof(flow.ports.ports));
> >>>>>> +               break;
> >>>>>> +
> >>>>>> +       case VIRTIO_NET_HASH_REPORT_UDPv4:
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state,
> >>>>>> +                                        (__be32 *)&flow.addrs.v4addrs,
> >>>>>> +                                        sizeof(flow.addrs.v4addrs));
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state, &flow.ports.ports,
> >>>>>> +                                        sizeof(flow.ports.ports));
> >>>>>> +               break;
> >>>>>> +
> >>>>>> +       case VIRTIO_NET_HASH_REPORT_IPv6:
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state,
> >>>>>> +                                        (__be32 *)&flow.addrs.v6addrs,
> >>>>>> +                                        sizeof(flow.addrs.v6addrs));
> >>>>>> +               break;
> >>>>>> +
> >>>>>> +       case VIRTIO_NET_HASH_REPORT_TCPv6:
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state,
> >>>>>> +                                        (__be32 *)&flow.addrs.v6addrs,
> >>>>>> +                                        sizeof(flow.addrs.v6addrs));
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state, &flow.ports.ports,
> >>>>>> +                                        sizeof(flow.ports.ports));
> >>>>>> +               break;
> >>>>>> +
> >>>>>> +       case VIRTIO_NET_HASH_REPORT_UDPv6:
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state,
> >>>>>> +                                        (__be32 *)&flow.addrs.v6addrs,
> >>>>>> +                                        sizeof(flow.addrs.v6addrs));
> >>>>>> +               virtio_net_toeplitz_calc(&toeplitz_state, &flow.ports.ports,
> >>>>>> +                                        sizeof(flow.ports.ports));
> >>>>>> +               break;
> >>>>>> +
> >>>>>> +       default:
> >>>>>> +               hash->report = VIRTIO_NET_HASH_REPORT_NONE;
> >>>>>> +               return;
> >>>>>
> >>>>> So I still think we need a comment here to explain why this is not an
> >>>>> issue if the device can report HASH_XXX_EX. Or we need to add the
> >>>>> support, since this is the code from the driver side, I don't think we
> >>>>> need to worry about the device implementation issues.
> >>>>
> >>>> This is on the device side, and don't report HASH_TYPE_XXX_EX.
> >>>>
> >>>>>
> >>>>> For the issue of the number of options, does the spec forbid fallback
> >>>>> to VIRTIO_NET_HASH_REPORT_NONE? If not, we can do that.
> >>>>
> >>>> 5.1.6.4.3.4 "IPv6 packets with extension header" says:
> >>>>    > If VIRTIO_NET_HASH_TYPE_TCP_EX is set and the packet has a TCPv6
> >>>>    > header, the hash is calculated over the following fields:
> >>>>    > - Home address from the home address option in the IPv6 destination
> >>>>    >   options header. If the extension header is not present, use the
> >>>>    >   Source IPv6 address.
> >>>>    > - IPv6 address that is contained in the Routing-Header-Type-2 from the
> >>>>    >   associated extension header. If the extension header is not present,
> >>>>    >   use the Destination IPv6 address.
> >>>>    > - Source TCP port
> >>>>    > - Destination TCP port
> >>>>
> >>>> Therefore, if VIRTIO_NET_HASH_TYPE_TCP_EX is set, the packet has a TCPv6
> >>>> and an home address option in the IPv6 destination options header is
> >>>> present, the hash is calculated over the home address. If the hash is
> >>>> not calculated over the home address in such a case, the device is
> >>>> contradicting with this section and violating the spec. The same goes
> >>>> for the other HASH_TYPE_XXX_EX types and Routing-Header-Type-2.
> >>>
> >>> Just to make sure we are one the same page. I meant:
> >>>
> >>> 1) If the hash is not calculated over the home address (in the case of
> >>> IPv6 destination destination), it can still report
> >>> VIRTIO_NET_RSS_HASH_TYPE_IPv6. This is what you implemented in your
> >>> series. So the device can simply fallback to e.g TCPv6 if it can't
> >>> understand all or part of the IPv6 options.
> >>
> >> The spec says it can fallback if "the extension header is not present",
> >> not if the device can't understand the extension header.
> >
> > I don't think so,
> >
> > 1) spec had a condition beforehand:
> >
> > """
> > If VIRTIO_NET_HASH_TYPE_TCP_EX is set and the packet has a TCPv6
> > header, the hash is calculated over the following fields:
> > ...
> > If the extension header is not present ...
> > """
> >
> > So the device can choose not to set VIRTIO_NET_HASH_TYPE_TCP_EX as
> > spec doesn't say device MUST set VIRTIO_NET_HASH_TYPE_TCP_EX if ...
> >
> > 2) implementation wise, since device has limited resources, we can't
> > expect the device can parse arbitrary number of ipv6 options
> >
> > 3) if 1) and 2) not the case, we need fix the spec otherwise implement
> > a spec compliant device is impractical
>
> The statement is preceded by the following:
>  >  The device calculates the hash on IPv4 packets according to
>  > ’Enabled hash types’ bitmask as follows:
>
> The 'Enabled hash types' bitmask is specified by the device.
>
> I think the spec needs amendment.

Michael, can you help to clarify here?

>
> I wonder if there are any people interested in the feature though.
> Looking at virtnet_set_hashflow() in drivers/net/virtio_net.c, the
> driver of Linux does not let users configure HASH_TYPE_XXX_EX. I suppose
> Windows supports HASH_TYPE_XXX_EX, but those who care network
> performance most would use Linux so HASH_TYPE_XXX_EX support without
> Linux driver's support may not be useful.

It might be still interesting for example for the hardware virtio
vendors to support windows etc.

>
> >
> >>
> >>> 2) the VIRTIO_NET_SUPPORTED_HASH_TYPES is not checked against the
> >>> tun_vnet_ioctl_sethash(), so userspace may set
> >>> VIRTIO_NET_HASH_TYPE_TCP_EX regardless of what has been returned by
> >>> tun_vnet_ioctl_gethashtypes(). In this case they won't get
> >>> VIRTIO_NET_HASH_TYPE_TCP_EX.
> >>
> >> That's right. It's the responsibility of the userspace to set only the
> >> supported hash types.
> >
> > Well, the kernel should filter out the unsupported one to have a
> > robust uAPI. Otherwise, we give green light to the buggy userspace
> > which will have unexpected results.
>
> My reasoning was that it may be fine for some use cases other than VM
> (e.g., DPDK); in such a use case, it is fine as long as the UAPI works
> in the best-effort basis.

Best-effort might increase the chance for user visisable changes after
migration.

>
> For example, suppose a userspace program that processes TCP packets; the
> program can enable: HASH_TYPE_IPv4, HASH_TYPE_TCPv4, HASH_TYPE_IPv6, and
> HASH_TYPE_TCPv6. Ideally, the kernel should support all the hash types,
> but, even if e.g., HASH_TYPE_TCPv6 is not available,

For "available" did you mean it is not supported by the device?

> it will fall back
> to HASH_TYPE_IPv6, which still does something good and may be acceptable.

This fallback is exactly the same as I said above, let
VIRTIO_NET_HASH_TYPE_TCP_EX to fallback.

My point is that, the implementation should either:

1) allow fallback so it can claim to support all hash types

or

2) don't allow fallback so it can only support a part of the hash types

If we're doing something in the middle, for example, allow part of the
type to fallback.

>
> That said, for a use case that involves VM and implements virtio-net
> (e.g., QEMU), setting an unsupported hash type here is definitely a bug.
> Catching the bug may outweigh the extra trouble for other use cases.
>
> >
> >>
> >>> 3) implementing part of the hash types might complicate the migration
> >>> or at least we need to describe the expectations of libvirt or other
> >>> management in this case. For example, do we plan to have a dedicated
> >>> Qemu command line like:
> >>>
> >>> -device virtio-net-pci,hash_report=on,supported_hash_types=X,Y,Z?
> >>
> >> I posted a patch series to implement such a command line for vDPA[1].
> >> The patch series that wires this tuntap feature up[2] reuses the
> >> infrastructure so it doesn't bring additional complexity.
> >>
> >> [1]
> >> https://lore.kernel.org/qemu-devel/20250530-vdpa-v1-0-5af4109b1c19@daynix.com/
> >> [2]
> >> https://lore.kernel.org/qemu-devel/20250530-hash-v5-0-343d7d7a8200@daynix.com/
> >
> > I meant, if we implement a full hash report feature, it means a single
> > hash cmdline option is more than sufficient and so compatibility code
> > can just turn it off when dealing with machine types. This is much
> > more simpler than
> >
> > 1) having both hash as well as supported_hash_features
> > 2) dealing both hash as well as supported_hash_features in compatibility codes
> > 3) libvirt will be happy
> >
> > For [1], it seems it introduces a per has type option, this seems to
> > be a burden to the management layer as it need to learn new option
> > everytime a new hash type is supported
>
> Even with the command line you proposed (supported_hash_types=X,Y,Z), it
> is still necessary to know the values the supported_hash_types property
> accepts (X.Y,Z), so I don't think it makes difference.

It could be a uint32_t.

>
> The burden to the management layer is already present for features, so
> it is an existing problem (or its mere extension).

Yes, but since this feature is new it's better to try our best to avoid that.

>
> This problem was discussed in the following thread in the past, but no
> solution is implemented yet, and probably solving it will be difficult.
> https://lore.kernel.org/qemu-devel/20230731223148.1002258-5-yuri.benditovich@daynix.com/

It's a similar issue but not the same, it looks more like a discussion
on whether the fallback from vhost-net to qemu works for missing
features etc.

Thanks

>
> Regards,
> Akihiko Odaki
>