Re: [PATCH v21] virtio-net: support inner header hash

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jul 03 2023, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Jul 03, 2023 at 11:27:11PM +0800, Heng Qi 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>
>
> Cornelia last minute comments before I start a vote?

Please just go ahead.