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

[Dragos Tatulea <dtatulea@nvidia.com>]

On 24.08.26 17:32, Luigi Rizzo wrote:
> On Mon, Aug 24, 2026 at 10:59 AM Dragos Tatulea <dtatulea@nvidia.com> wrote:
>>
> ...
>> An example of this is Jiri's system_cc_shared heap which is a dma-buf
>> heap with decrypted memory for userspace.
>>
>>> I am still looking into this, I was planning to bring this up in the
>>> upcoming LPC.
>>> I will give this patch a try. However, I believe that we need a more
>>> generalised concept for CoCo pre-decrypted allocators in the kernel.
>>>
>> There is a talk at LPC in the networking track about this [2]. This is
>> exactly the type of discussion that I was hoping to have there.
>>
>> Besides the issues mentioned in this thread we've also found that a lot
>> of overhead can come only from swiotlb allocations when running many queues.
>>
>> I will add information about this series in my talk. Hopefully I will also
>> have time to add some numbers for comparison.
>>
>> Sorry for the late reply but I spotted this thread only now by
>> accident.
>>
>> [1] https://lore.kernel.org/all/20260325192352.437608-3-jiri@resnulli.us/
>> [2] https://lpc.events/event/20/contributions/2464
> 
> Thank you for the feedback and links.
> 
> Do you have some code describing [2] ?
> 
Not yet I will send a link once I push it somewhere. But the idea is simple:
a driver side alternative data-path which uses pre-allocated coherent DMA buffers.
This doesn't make memcpy go away but eliminates the swiotlb all together and it
reduces the DMA map/unmap overhead.

> I just sent a cleaned-up v2 of my previous series, if you want to try
> it please use the latter.
>
Thanks! Had a quick lock and wrote some comments.

Thanks,
Dragos