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

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Message-ID: <Z6Jt62bZEeHnN1JP@mini-arch>
Date: Tue, 4 Feb 2025 11:43:39 -0800
From: Stanislav Fomichev <stfomichev@...il.com>
To: Mina Almasry <almasrymina@...gle.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 02/04, Mina Almasry wrote:
> 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?

I don't run into any issues with those unmaps in place, but I'm running x86
with iommu bypass (and as you mention in the other thread, those
calls are no-ops in this case).