linux-kernel - Re: [RFC] User space to kernel space copy optimization

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Message-ID: <1447788530.83864.217.camel@intel.com>
Date:	Tue, 17 Nov 2015 19:28:50 +0000
From:	"Jiang, Dave" <dave.jiang@...el.com>
To:	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"okaya@...eaurora.org" <okaya@...eaurora.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] User space to kernel space copy optimization

On Mon, 2015-11-16 at 23:19 +0000, Sinan Kaya wrote:
> On 11/16/2015 5:57 PM, Jiang, Dave wrote:
> > > One of the things I'm interested in is to use a memcpy capable
> > > DMA
> > > > engine HW to optimize user space and kernel space parameter
> > > > copying.
> > Have you looked at why NET_DMA was deprecated and using DMA engine
> > to
> > do kernel->user copy could be a problem?
> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/com
> > mit/
> > ?id=77873803363c
> > 
> 
> As far as I know, the problem was the TCP/IP stack trying to work on
> a
> buffer that was given to the DMA engine for moving. It was causing
> stale
> data problems.
> 
> The goal was to let DMA engine move the data while TCP/IP stack was
> working on something else. Then, synchronize on a known point. Of
> course, when above violation happens; things break.
> 
> Are you expecting a similar problem on user space to kernel space
> interaction? I think the ownership buffers are pretty much defined,
> isn't it?

No. I thought you were investigating both ways so thought you might
need to be aware. 

> 
> 
> -- 
> Sinan Kaya
> Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center,
> Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
> Linux Foundation Collaborative Project