[virtio-comment] Re: [PATCH v19] virtio-net: support inner header hash

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jun 28, 2023 at 11:46:22AM +0800, Jason Wang wrote:
> On Wed, Jun 28, 2023 at 12:35 AM Heng Qi <hengqi@linux.alibaba.com> wrote:
> >
> > 1. Currently, a received encapsulated packet has an outer and an inner header, but
> > the virtio device is unable to calculate the hash for the inner header. The same
> > flow can traverse through different tunnels, resulting in the encapsulated
> > packets being spread across multiple receive queues (refer to the figure below).
> > However, in certain scenarios, we may need to direct these encapsulated packets of
> > the same flow to a single receive queue. This facilitates the processing
> > of the flow by the same CPU to improve performance (warm caches, less locking, etc.).
> >
> >                client1                    client2
> >                   |        +-------+         |
> >                   +------->|tunnels|<--------+
> >                            +-------+
> >                               |  |
> >                               v  v
> >                       +-----------------+
> >                       | monitoring host |
> >                       +-----------------+
> >
> > To achieve this, the device can calculate a symmetric hash based on the inner headers
> > of the same flow.
> >
> > 2. For legacy systems, they may lack entropy fields which modern protocols have in
> > the outer header, resulting in multiple flows with the same outer header but
> > different inner headers being directed to the same receive queue. This results in
> > poor receive performance.
> >
> > To address this limitation, inner header hash can be used to enable the device to advertise
> > the capability to calculate the hash for the inner packet, regaining better receive performance.
> >
> > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/173
> > Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > Reviewed-by: Parav Pandit <parav@nvidia.com>
> > ---
> > v18->v19:
> >         1. Have a single structure instead of two. @Michael S . Tsirkin
> >         2. Some small rewrites. @Michael S . Tsirkin
> >         3. Rebase to master.
> >
> > v17->v18:
> >         1. Some rewording suggestions from Michael (Thanks!).
> >         2. Use 0 to disable inner header hash and remove
> >            VIRTIO_NET_HASH_TUNNEL_TYPE_NONE.
> > v16->v17:
> >         1. Some small rewrites. @Parav Pandit
> >         2. Add Parav's Reviewed-by tag (Thanks!).
> >
> > v15->v16:
> >         1. Remove the hash_option. In order to delimit the inner header hash and RSS
> >            configuration, the ability to configure the outer src udp port hash is given
> >            to RSS. This is orthogonal to inner header hash, which will be done in the
> >            RSS capability extension topic (considered as an RSS extension together
> >            with the symmetric toeplitz hash algorithm, etc.). @Parav Pandit @Michael S . Tsirkin
> >         2. Fix a 'field' typo. @Parav Pandit
> >
> > v14->v15:
> >         1. Add tunnel hash option suggested by @Michael S . Tsirkin
> >         2. Adjust some descriptions.
> >
> > v13->v14:
> >         1. Move supported_hash_tunnel_types from config space into cvq command. @Parav Pandit
> 
> I may miss some discussions, but this complicates the provisioning a lot.
> 
> Having it in the config space, then a type agnostic provisioning
> through config space + feature bits just works fine.
> 
> If we move it only via cvq, we need device specific provisioning interface.
> 
> Thanks

Yea that's what I said too. Debugging too.  I think we should build a
consistent solution that allows accessing config space through DMA,
separately from this effort.  Parav do you think you can live with this
approach so this specific proposal can move forward?

-- 
MST


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