netdev - Re: [PATCH net-next v3 7/7] bnxt_en: add support for device memory tcp

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Message-ID: <CAHS8izPLyTa=rUbFo0B29HWHdmLV4rF4q3qC6XkgksGMSFxjyA@mail.gmail.com>
Date: Fri, 18 Oct 2024 11:25:30 +0300
From: Mina Almasry <almasrymina@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Jakub Kicinski <kuba@...nel.org>, Leon Romanovsky <leonro@...dia.com>, 
	Christian König <christian.koenig@....com>, 
	Samiullah Khawaja <skhawaja@...gle.com>, Taehee Yoo <ap420073@...il.com>, davem@...emloft.net, 
	pabeni@...hat.com, edumazet@...gle.com, netdev@...r.kernel.org, 
	linux-doc@...r.kernel.org, donald.hunter@...il.com, corbet@....net, 
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	willemb@...gle.com, aleksander.lobakin@...el.com, dw@...idwei.uk, 
	sridhar.samudrala@...el.com, bcreeley@....com
Subject: Re: [PATCH net-next v3 7/7] bnxt_en: add support for device memory tcp

On Tue, Oct 15, 2024 at 3:44 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Tue, Oct 15, 2024 at 04:10:44AM +0300, Mina Almasry wrote:
> > On Tue, Oct 15, 2024 at 3:16 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Tue, 15 Oct 2024 01:38:20 +0300 Mina Almasry wrote:
> > > > Thanks Jason. In that case I agree with Jakub we should take in his change here:
> > > >
> > > > https://lore.kernel.org/netdev/20241009170102.1980ed1d@kernel.org/
> > > >
> > > > With this change the driver would delegate dma_sync_for_device to the
> > > > page_pool, and the page_pool will skip it altogether for the dma-buf
> > > > memory provider.
> > >
> > > And we need a wrapper for a sync for CPU which will skip if the page
> > > comes from an unreadable pool?
> >
> > This is where it gets a bit tricky, no?
> >
> > Our production code does a dma_sync_for_cpu but no
> > dma_sync_for_device. That has been working reliably for us with GPU
>
> Those functions are all NOP on systems you are testing on.
>

OK, thanks. This is what I wanted to confirm. If you already know this
here then there is no need to wait for me to confirm.

> The question is what is correct to do on systems where it is not a
> NOP, and none of this is really right, as I explained..
>
> > But if you or Jason think that enforcing the 'no dma_buf_sync_for_cpu'
> > now is critical, no problem. We can also provide this patch, and seek
> > to revert it or fix it up properly later in the event it turns out it
> > causes issues.
>
> What is important is you organize things going forward to be able to
> do this properly, which means the required sync type is dependent on
> the actual page being synced and you will eventually somehow learn
> which is required from the dmabuf.
>
> Most likely nobody will ever run this code on system where dma_sync is
> not a NOP, but we should still use the DMA API properly and things
> should make architectural sense.
>

Makes sense. OK, we can do what Jakub suggested in the thread earlier.
I.e. likely some wrapper which skips the dma_sync_for_cpu if the
netmem is unreadable.

--
Thanks,
Mina