linux-kernel - Re: [PATCH 0/10] x86: handle HW IOMMU initialization failure gracely

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Message-ID: <20091028132119.GC5876@amd.com>
Date:	Wed, 28 Oct 2009 14:21:19 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	linux-kernel@...r.kernel.org, chrisw@...s-sol.org,
	dwmw2@...radead.org, mingo@...e.hu
Subject: Re: [PATCH 0/10] x86: handle HW IOMMU initialization failure
 gracely

On Wed, Oct 28, 2009 at 03:47:34PM +0900, FUJITA Tomonori wrote:
> If HW IOMMU initialization fails (Intel VT-d often does typically due
> to BIOS bugs), we fall back to nommu. It doesn't work for the majority
> since nowadays we have more than 4GB memory so we must use swiotlb
> instead of nommu.
> 
> The problem is that it's too late to initialize swiotlb when HW IOMMU
> initialization fails. We need to allocate swiotlb memory earlier from
> bootmem allocator. Chris explained the issue in detail:
> 
> http://marc.info/?l=linux-kernel&m=125657444317079&w=2
> 
> 
> The current x86 IOMMU initialization sequence is too complicated and
> handling the above issue makes it more hacky.
> 
> This series changes x86 IOMMU initialization sequence to handle the
> above issue cleanly.
> 
> The new x86 IOMMU initialization sequence are:
> 
> 1. initializing swiotlb (and setting swiotlb to 1) in the case of
> (max_pfn > MAX_DMA32_PFN && !no_iommu). dma_ops is set to
> swiotlb_dma_ops or nommu_dma_ops.
> 
> 2. calling the detection functions of all the IOMMUs
> 
> 3. the detection function sets x86_init.iommu.iommu_init to the IOMMU
> initialization function (so we can avoid calling the initialization
> functions of all the IOMMUs needlessly).
> 
> 4. if the IOMMU initialization function doesn't need to swiotlb then
> sets swiotlb to zero (e.g. the initialization is sucessful).
> 
> 5. if we find that swiotlb is set to zero, we free swiotlb resource.

I started to test this patchset. It breaks the iommu=soft selection to
force using swiotlb usage. On my machine always the AMD IOMMU driver
gots selected and initialized.

	Joerg


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