Re: [PATCH] swiotlb: avoid double copy with swiotlb on tx socket

[Luigi Rizzo]

On Tue, Jun 16, 2026 at 2:25 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Mon, 15 Jun 2026 23:42:20 +0000 Luigi Rizzo wrote:
> > The use of swiotlb causes an extra data copy on I/O.  For tx sockets,
> > especially with greedy senders, this has a high chance of happening in
> > the softirq handler for tx network interrupts, creating a significant
> > performance bottleneck.
>
> What's the use case? I associate swiotlb with debug / testing mostly,
> so it'd be useful for people like me to explain why you care.

Ah sorry, I forgot to mention.
swiotlb is used in guest kernels for confidential computing VMs.
Ordinary memory pages are encrypted and the host or devices
have no way to decrypt them, so the kernel must use
unencrypted bounce buffers to exchange data with I/O devices.

cheers
luigi