linux-kernel - Re: [PATCH 00 of 14] swiotlb/x86: lay groundwork for xen dom0 use of swiotlb

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Date:	Wed, 17 Dec 2008 08:31:43 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, x86@...nel.org,
	ian.campbell@...rix.com, jbeulich@...ell.com, joerg.roedel@....com
Subject: Re: [PATCH 00 of 14] swiotlb/x86: lay groundwork for xen dom0 use
 of swiotlb

FUJITA Tomonori wrote:
> On Tue, 16 Dec 2008 21:35:13 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
>
>   
>> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>
>>     
>>> Hi Ingo,
>>>
>>> Here's some patches to clean up swiotlb to prepare for some Xen dom0 
>>> patches.  These have been posted before, but undergone a round of 
>>> cleanups to address various comments.
>>>       
>> applied to tip/core/iommu, thanks Jeremy.
>>
>> the only patch that seems to have the potential to break drivers is:
>>
>>   be4ac7b: swiotlb: consistently use address_needs_mapping everywhere
>>     
>
> Yeah, as I already wrote, this patch is wrong.
>   

I'll have a look.

> I think that the whole patchset is against the swiotlb design. swiotlb
> is designed to be used as a library. Each architecture implements the
> own swiotlb by using swiotlb library
> (e.g. arch/x86/kernel/pci-swiotlb_64.c).
>   

The whole patchset?  The bulk of the changes to lib/swiotlb.c are 
relatively minor to remove the unwarranted assumptions it is making in 
the face of a new user.  They will have no effect on other existing 
users, including non-Xen x86 builds.

If you have specific objections we can discuss those, but I don't think 
there's anything fundamentally wrong with making lib/swiotlb.c a bit 
more generically useful.

> For example, adding the following code (9/14) for just Xen that the
> majority of swiotbl users (x86_64 and IA64) don't need to the library
> is against the design.
>   

If the architecture doesn't support highmem then this code will compile 
to nothing - PageHighMem() will always evaluate to 0.  It will therefore 
have zero effect on the code generated for IA64 or x86-64.  This is not 
really a Xen-specific change, but a result of adding swiotlb support for 
i386.  Other architectures which support a notion of highmem would also 
need this code if they wanted to use swiotlb.

    J
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