Re: [PATCH 0/3] net/9p: Add 9P transport over USB4STREAM

[Dominique Martinet <asmadeus@codewreck.org>]

Alan Borzeszkowski wrote on Thu, Sep 24, 2026 at 06:23:30PM +0200:
> This series adds trans=thunderbolt, a 9P transport over the
> Thunderbolt/USB4 DMA tunnels exposed by the tbstream character devices
> (USB4STREAM). It's designed for simple host-to-host file sharing over
> single Thunderbolt/USB4 cable.
> 
> Today the only practical way to use 9P over a USB4 link is trans=fd
> which, due to its design, cannot fully utilize USB4STREAM capabilities.
> Thunderbolt transport offers a simpler mount (no fd redirection required),
> busy-poll support and substantial throughput increase compared to FD
> transport.
> 
> User may mount 9P export with following command:
> 
> mount -t 9p -o trans=thunderbolt,aname=<exported_remote> /dev/tbstreamX /mnt
> 
> Thunderbolt transport achieves a median 1.21x more throughput compared
> to FD transport under same conditions, and the advantage grows with
> concurrency: 1.03-1.09x at 1-2 threads, 1.14-1.27x at 4, and 1.34-2.28x
> at 8-16.
> 
> As an example, 4K random reads, 4 worker threads. FD transport achieves
> ~53k IOPS, Thunderbolt transport ~66k (+24.7%). 4K random writes follow:
> FD ~28k IOPS, Thunderbolt ~33k (+19.4%).

Thanks for the patches

We've kept adding new transports for 9p lately but maintenance is
becoming a burden so let me push back a bit: is the increased
performance really worth it over trans=fd ?

If this could be used to bringup a system with a root filesystem like it
was done for trans=usbg I'd see the point, but otherwise the simpler
usage could be addressed with a simple mount.9p helper, and at this
point I'd consider usb4stream to be used for perhaps one-shot copy and
not care too much about 4k random IOs (so less affected by latency
if msize is large enough, and for a one-shot backup it doesn't matter
too much if it takes 6 instead of 5 minutes)

If you can convince me this is worth it and can take care of the
transport for the forseable future I'm not dead set to refuse this, but
I can't say I'm convinced right now, especially as I can't even test
this (I don't own two computers with working usb-c...)

Thanks,
-- 
Dominique Martinet | Asmadeus