On Thu, 20 Apr 2023 07:13:49 -0700, Jakub Kicinski <kuba at kernel.org>
wrote:> On Wed, 19 Apr 2023 23:19:22 -0700 Christoph Hellwig wrote:
> > > In this case yes, pinned user memory, it gets sliced up into MTU
sized
> > > chunks, fed into an Rx queue of a device, and user can see
packets
> > > without any copies.
> >
> > How long is the life time of these mappings? Because dma_map_*
> > assumes a temporary mapping and not one that is pinned bascically
> > forever.
>
> Yeah, this one is "for ever".
>
> > > Quite similar use case #2 is upcoming io_uring / "direct
placement"
> > > patches (former from Meta, latter for Google) which will try to
receive
> > > just the TCP data into pinned user memory.
> >
> > I don't think we can just long term pin user memory here. E.g.
for
> > confidential computing cases we can't even ever do DMA straight to
> > userspace. I had that conversation with Meta's block folks who
> > want to do something similar with io_uring and the only option is an
> > an allocator for memory that is known DMAable, e.g. through dma-bufs.
> >
> > You guys really all need to get together and come up with a scheme
> > that actually works instead of piling these hacks over hacks.
>
> Okay, that simplifies various aspects. We'll just used dma-bufs from
> the start in the new APIs.
I am not particularly familiar with dma-bufs. I want to know if this mechanism
can solve the problem of virtio-net.
I saw this framework, allowing the driver do something inside the ops of
dma-bufs.
If so, is it possible to propose a new patch based on dma-bufs?
Thanks.