linux-kernel - Re: [PATCH 2/2] x86 amd_gart_64: Verify we can perform the remapping requested

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Message-ID: <20111112090203.GD11779@8bytes.org>
Date:	Sat, 12 Nov 2011 10:02:03 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86 amd_gart_64: Verify we can perform the
	remapping requested

On Fri, Nov 11, 2011 at 10:59:33AM -0800, Eric W. Biederman wrote:
> Joerg Roedel <joro@...tes.org> writes:
> > Okay, thinking more about this, I am not so sure anymore it is a really
> > good idea to fix it this way. It basically signals the driver that DMA
> > is not possible with the device if the aperture does not fit into
> > the dma_mask. But DMA within the ZONE_DMA might still work, no?
> >
> > So I think the right way to fix it is to return a dma-mapping error if
> > and only if the address needs to be remapped by the aperture (of swiotlb or
> > gart).
> 
> Perhaps I am mistaken but since the method is iommu type specific if we
> don't actually use the iommu we should not call the method so I think
> this is patch is already implementing what you want.

No it does not. With GART or SWIOTLB all DMA-mapping functions call
directly into the IOMMU driver. The driver then checks whether it needs
to remap or not (see dma_capable() in SWIOTLB or need_iommu() in GART).

I think the check you want to add needs to happen after these checks in
each of the IOMMU drivers.


	Joerg

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