Re: [PATCH net-next v9 1/6] virtio_net: Add functions for hashing

[Jason Wang <jasowang@redhat.com>]

On Tue, Mar 11, 2025 at 1:49 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> On 2025/03/11 9:47, Jason Wang wrote:
> > On Mon, Mar 10, 2025 at 2:53 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> >>
> >> On 2025/03/10 12:55, Jason Wang wrote:
> >>> On Fri, Mar 7, 2025 at 7:01 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 02a9f4dc594d02372a6c1850cd600eff9d000d8d..426f33b4b82440d61b2af9fdc4c0b0d4c571b2c5 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)
> >>>
> >>> Let's explain why
> >>>
> >>> #define VIRTIO_NET_HASH_REPORT_IPv6_EX         7
> >>> #define VIRTIO_NET_HASH_REPORT_TCPv6_EX        8
> >>> #define VIRTIO_NET_HASH_REPORT_UDPv6_EX        9
> >>>
> >>> are missed here.
> >>
> >> Because they require parsing IPv6 options and I'm not sure how many we
> >> need to parse. QEMU's eBPF program has a hard-coded limit of 30 options;
> >> it has some explanation for this limit, but it does not seem definitive
> >> either:
> >> https://gitlab.com/qemu-project/qemu/-/commit/f3fa412de28ae3cb31d38811d30a77e4e20456cc#6ec48fc8af2f802e92f5127425e845c4c213ff60_0_165
> >>
> >
> > How about the usersapce datapath RSS in Qemu? (We probably don't need
> > to align with eBPF RSS as it's just a reference implementation)
>
> The userspace datapath RSS has no limit.
>
> The reference implementation is the userspace datapath. The eBPF program
>   is intended to bring real performance benefit to Windows guests in
> contrary.
>
> The userspace implementation does its best to provide defined RSS
> capabilities but may not be performant. Parsing all IPv6 options have a
> performance implication, but it is fine because it is not intended to be
> performant in the first place.
>
> The performance problem is inherent to the userspace implementation,
> which adds an extra overhead to the datapath. The eBPF program on the
> other hand does not incur such overhead because it replaces the existing
> steering algorithm (automq) instead of adding another layer. Hence the
> eBPF program can be practical.
>
> That said, it is not that important to align with the userspace and eBPF
> RSS in QEMU because they are still experimental anyway; the eBPF RSS has
> potential to become a practical implementation but it is still in
> development. The libvirt integration for the eBPF RSS is still not
> complete, and we occasionally add fixes for RSS and hash reporting
> without backporting to the stable branch.
>
> I'm adding interfaces to negotiate hash types rather for the future
> extensibility. The specification may gain more hash types in the future
> and other vhost backends may have a different set of hash types
> supported. Figuring out how to deal with different sets of supported
> hash typs is essential for both the kernel and QEMU.
>
> >
> >> In this patch series, I add an ioctl to query capability instead; it
> >> allows me leaving those hash types unimplemented and is crucial to
> >> assure extensibility for future additions of hash types anyway. Anyone
> >> who find these hash types useful can implement in the future.
> >
> > Yes, but we need to make sure no userspace visible behaviour changes
> > after migration.
>
> Indeed, the goal is to make extensibility and migration compatible.

So I see this part:

+ uint32_t supported_hash_types = n->rss_data.supported_hash_types;
+ uint32_t peer_hash_types = n->rss_data.peer_hash_types;
+ bool use_own_hash =
+ (supported_hash_types & VIRTIO_NET_RSS_SUPPORTED_HASHES) ==
+ supported_hash_types;
+ bool use_peer_hash =
+ n->rss_data.peer_hash_available &&
+ (supported_hash_types & peer_hash_types) == supported_hash_types;

It looks like it would be a challenge to support vhost-user in the
future if vhost-user supports hash feature others than source?

>
> >
> >>
> >>>
> >>> And explain how we could maintain migration compatibility
> >>>
> >>> 1) Does those three work for userspace datapath in Qemu? If yes,
> >>> migration will be broken.
> >>
> >> They work for userspace datapath so my RFC patch series for QEMU uses
> >> TUNGETVNETHASHCAP to prevent breaking migration:
> >> https://patchew.org/QEMU/20240915-hash-v3-0-79cb08d28647@daynix.com/
> >>
> >
> > Ok, let's mention this in the cover letter. Another interesting thing
> > is the migration from 10.0 to 9.0.
>
> The patch series is already mentioned in the cover letter. A description
> of the intended use case of TUNGETVNETHASHCAP will be a good addition.
> I'll add it to this patch so that it will be kept in tree after it gets
> merged.
>
> Migration between two different QEMU versions should be handled with
> versioned machine types.
>
> When a machine created in 9.0 is being migrated to 10.0, the machine
> must set the hash type properties to match with the hash types supported
> by the existing implementations, which means it sets the property for
> VIRTIO_NET_HASH_REPORT_IPv6_EX to true, for example. Because this hash
> type is currently not included in TUNGETVNETHASHCAP, the machine will
> keep using the implementation used previously. The machine can be also
> migrated back to 9.0 again.
>
> A machine type with version 10.0 cannot be migrated to 9.0 by design so
> there is no new problem.

I meant migrate qemu 11.0 with machine type 10.0 to qemu 10.0 with
machine 10.0 etc.

>
> >
> >> This patch series first adds configuration options for users to choose
> >> hash types. QEMU then automatically picks one implementation from the
> >> following (the earlier one is the more preferred):
> >> 1) The hash capability of vhost hardware
> >> 2) The hash capability I'm proposing here
> >> 3) The eBPF program
> >> 4) The pure userspace implementation
> >>
> >> This decision depends on the following:
> >> - The required hash types; supported ones are queried for 1) and 2)
> >> - Whether vhost is enabled or not and what vhost backend is used
> >> - Whether hash reporting is enabled; 3) is incompatible with this
> >>
> >> The network device will not be realized if no implementation satisfies
> >> the requirements.
> >
> > This makes sense, let's add this in the cover letter.
>
> I'll add it to the QEMU patch as it's more about details of QEMU.
> The message of this patch will explain how TUNGETVNETHASHCAP and
> TUNSETVNETHASH makes extensibility and migrattion compatible in general.
>
> Regards,
> Akihiko Odaki
>
> >
> >>
> >>> 2) once we support those three in the future. For example, is the qemu
> >>> expected to probe this via TUNGETVNETHASHCAP in the destination and
> >>> fail the migration?
> >>
> >> QEMU is expected to use TUNGETVNETHASHCAP, but it can selectively enable
> >> hash types with TUNSETVNETHASH to keep migration working.
> >>
> >> In summary, this patch series provides a sufficient facility for the
> >> userspace to make extensibility and migration compatible;
> >> TUNGETVNETHASHCAP exposes all of the kernel capabilities and
> >> TUNSETVNETHASH allows the userspace to limit them.
> >>
> >> Regards,
> >> Akihiko Odaki
> >
> > Fine.
> >
> > Thanks
> >

Thanks