Re: RFC - Tap io_uring PMD

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Stephen,

On 10/30/24 22:56, Stephen Hemminger wrote:
> The current tap device is slow both due to architectural choices and the
> overhead of Linux system calls. I am exploring a how to fix that but some
> of the choices require some tradeoffs. Which leads to some open questions:
> 
> 1. DPDK tap also support tunnel (TUN) mode where there is no Ethernet header
>     only L3. Does anyone actually use this? It is different than what every other
>     PMD expects.
> 
> 2. The fastest way to use kernel TAP device would be to use io_uring.
>     But this was added in 5.1 kernel (2019). Rather than having conditional or
>     dual mode in DPDK tap device, perhaps there should just be a new PMD tap_uring?
> 
> 3. Current TAP device provides hooks for several rte_flow types by playing
>     games with kernel qdisc. Does anyone really use this? Propose just not doing
>     this in new tap_uring.
> 
> 4. What other features of TAP device beyond basic send/receive make sense?
>     It looks like new device could support better statistics.
> 
> 5. What about Rx interrupt support?
> 
> Probably the hardest part of using io_uring is figuring out how to collect
> completions. The simplest way would be to handle all completions rx and tx
> in the rx_burst function.
> 

Why not just use Virtio-user PMD with Vhost-kernel backend [0]?
Are there any missing features that io_uring can address?

Regards,
Maxime

[0]: http://doc.dpdk.org/guides/howto/virtio_user_as_exception_path.html