netdev - Re: [PATCH net-next v3 0/6] Device memory TCP TX

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Message-ID: <CAHS8izNXo1cQmA5GijE-UW2X1OU6irMV9FRevL5tZW3B5NQ8rA@mail.gmail.com>
Date: Tue, 4 Feb 2025 10:38:05 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux.dev, linux-kselftest@...r.kernel.org, 
	Donald Hunter <donald.hunter@...il.com>, Jakub Kicinski <kuba@...nel.org>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Simon Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>, Andrew Lunn <andrew+netdev@...n.ch>, 
	Neal Cardwell <ncardwell@...gle.com>, David Ahern <dsahern@...nel.org>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Stefan Hajnoczi <stefanha@...hat.com>, Stefano Garzarella <sgarzare@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	sdf@...ichev.me, asml.silence@...il.com, dw@...idwei.uk, 
	Jamal Hadi Salim <jhs@...atatu.com>, Victor Nogueira <victor@...atatu.com>, 
	Pedro Tammela <pctammela@...atatu.com>, Samiullah Khawaja <skhawaja@...gle.com>
Subject: Re: [PATCH net-next v3 0/6] Device memory TCP TX

On Tue, Feb 4, 2025 at 10:06 AM Stanislav Fomichev <stfomichev@...il.com> wrote:
>
> On 02/04, Mina Almasry wrote:
> > On Tue, Feb 4, 2025 at 4:32 AM Paolo Abeni <pabeni@...hat.com> wrote:
> > >
> > > On 2/3/25 11:39 PM, Mina Almasry wrote:
> > > > The TX path had been dropped from the Device Memory TCP patch series
> > > > post RFCv1 [1], to make that series slightly easier to review. This
> > > > series rebases the implementation of the TX path on top of the
> > > > net_iov/netmem framework agreed upon and merged. The motivation for
> > > > the feature is thoroughly described in the docs & cover letter of the
> > > > original proposal, so I don't repeat the lengthy descriptions here, but
> > > > they are available in [1].
> > > >
> > > > Sending this series as RFC as the winder closure is immenient. I plan on
> > > > reposting as non-RFC once the tree re-opens, addressing any feedback
> > > > I receive in the meantime.
> > >
> > > I guess you should drop this paragraph.
> > >
> > > > Full outline on usage of the TX path is detailed in the documentation
> > > > added in the first patch.
> > > >
> > > > Test example is available via the kselftest included in the series as well.
> > > >
> > > > The series is relatively small, as the TX path for this feature largely
> > > > piggybacks on the existing MSG_ZEROCOPY implementation.
> > >
> > > It looks like no additional device level support is required. That is
> > > IMHO so good up to suspicious level :)
> > >
> >
> > It is correct no additional device level support is required. I don't
> > have any local changes to my driver to make this work. I think Stan
> > on-list was able to run the TX path (he commented on fixes to the test
> > but didn't say it doesn't work :D) and one other person was able to
> > run it offlist.
>
> For BRCM I had shared this: https://lore.kernel.org/netdev/ZxAfWHk3aRWl-F31@mini-arch/
> I have similar internal patch for mlx5 (will share after RX part gets
> in). I agree that it seems like gve_unmap_packet needs some work to be more
> careful to not unmap NIOVs (if you were testing against gve).

Hmm. I think you're right. We ran into a similar issue with the RX
path. The RX path worked 'fine' on initial merge, but it was passing
dmabuf dma-addrs to the dma-mapping API which Jason later called out
to be unsafe. The dma-mapping API calls with dmabuf dma-addrs will
boil down into no-ops for a lot of setups I think which is why I'm not
running into any issues in testing, but upon closer look, I think yes,
we need to make sure the driver doesn't end up passing these niov
dma-addrs to functions like dma_unmap_*() and dma_sync_*().

Stan, do you run into issues (crashes/warnings/bugs) in your setup
when the driver tries to unmap niovs? Or did you implement these
changes purely for safety?

Let me take a deeper look here and suggest something for the next
version. I think we may indeed need the driver to declare that it can
handle niovs in the TX path correctly (i.e. not accidentally pass niov
dma-addrs to the dma-mapping API).

--
Thanks,
Mina