netdev - Re: [PATCH v2] net: Allow no-cache copy from user on transmit

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Date:	Wed, 23 Mar 2011 12:48:05 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Michał Mirosław <mirqus@...il.com>,
	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: Allow no-cache copy from user on transmit

> Isn't nocache DMA a function of the I/O architecture not a function
> of the device driver? Shouldn't it be handled at PCI level somehow
> with considerations of CPU arch and quirks? Doesn't it make sense for
> non-network traffic as well.
>
> Hate to hold up a good optimization while waiting for a general
> solution, but commiting to an API prematurely would be bad as well.
>

There's an implicit assumption in the patch that if somewhere below
the copyfromuser the data is touched it would be more efficient to
copy through the cache than bypass it.  Whether the data is touched is
an attribute of the device and hence the per device control.

Now that I think about it, I don't really know what the actual
performance impact is to bypass cache copy and then touch the data.
I'd be interested to know if anyone has data on that, else I'll try to
contrive some benchmark numbers.

Tom

>
>
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