linux-kernel - Re: Question about iommu on x86

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Message-ID: <6d6b404ccb4dc0a713e8e12403acde32@admin.gogi.tv>
Date:	Wed, 25 May 2011 15:51:26 +0100
From:	Daniel Haid <d.haid@...i.tv>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: Question about iommu on x86_64 and radeon driver.

>> I see. Now, maybe this will not work for some reason, but
>> would it be possible to somehow disable both IOMMU and SWIOTLB
>> and tell all drivers only to do DMA with the lower 4GB of memory?
>
> Only if you allow 3GB or less in the machine. So you would have to do
> mem=3G as well.
>
> The reason is that (and you can see that yourself by looking at the
> E820), is that 1GB is actually _above_ the 4GB.

 I see. Is it possible to somehow give "mem=3G" to the kernel and still
 use the memory above 4G for something, like a ramdisk or something?
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