netdev - Re: [RFC net-next 1/4] net: Allow non parent devices to be used for ZC DMA

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Message-ID: <c5pxc7ppuizhvgasy57llo2domksote5uvo54q65shch3sqmkm@bgcnojnxt4hh>
Date: Wed, 2 Jul 2025 20:01:48 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: almasrymina@...gle.com, asml.silence@...il.com, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Saeed Mahameed <saeedm@...dia.com>, tariqt@...dia.com, cratiu@...dia.com, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC net-next 1/4] net: Allow non parent devices to be used for
 ZC DMA

On Wed, Jul 02, 2025 at 11:32:08AM -0700, Jakub Kicinski wrote:
> On Wed, 2 Jul 2025 20:24:23 +0300 Dragos Tatulea wrote:
> > For zerocopy (io_uring, devmem), there is an assumption that the
> > parent device can do DMA. However that is not always the case:
> > for example mlx5 SF devices have an auxiliary device as a parent.
> 
> Noob question -- I thought that the point of SFs was that you can pass
> them thru to a VM. How do they not have DMA support? Is it added on
> demand by the mediated driver or some such?
They do have DMA support. Maybe didn't state it properly in the commit
message. It is just that the the parent device
(sf_netdev->dev.parent.device) is not a DMA device. The grandparent
device is a DMA device though (PCI dev of parent PFs). But I wanted to
keep it generic. Maybe it doesn't need to be so generic?

Regarding SFs and VM passtrhough: my understanding is that SFs are more
for passing them to a container.

Thanks,
Dragos