linux-kernel - Re: [PATCH] x86: fix gart iommu using for amd 64 bit system

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Date:	Wed, 25 Nov 2009 08:25:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	yinghai@...nel.org, hpa@...or.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: fix gart iommu using for amd 64 bit system


* FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:

> On Tue, 24 Nov 2009 16:26:45 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:
> > 
> > > > don't think user will like to use SWIOTLB instead of swiotlb.
> > > 
> > > I doubt that users care about a way to fix their problems.
> > 
> > As far as swiotlb vs. GART goes, arguably a non-bouncing hw based 
> > solution (GART) is superior to a bouncing one (swiotlb), right?
> 
> Yeah, but GART is not a real IOMMU that always does address 
> translation. If users want performance, they need 64bit DMA capable 
> devices to skip GART. With such devices, GART and SWIOTLB are same.
> 
> So this hacky GART workaround is useful for only people who use non 
> 64bit DMA devices and broken BIOS with GART. IMO, it's not worth 
> having the workaround.

Well, what Yinghai is saying that for that (nonzero) subset of people 
this change is a regression, and his patch fixes that. We dont do 
regressions so this needs to be addressed.

Thanks,

	Ingo
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