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

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Message-Id: <20081218014653A.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 18 Dec 2008 01:51:14 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	jbeulich@...ell.com
Cc:	fujita.tomonori@....ntt.co.jp, joerg.roedel@....com,
	ian.campbell@...rix.com, mingo@...e.hu, jeremy@...p.org,
	x86@...nel.org, xen-devel@...ts.xensource.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00 of 14] swiotlb/x86: lay groundwork for xen dom0	
 useof swiotlb

On Wed, 17 Dec 2008 08:47:47 +0000
"Jan Beulich" <jbeulich@...ell.com> wrote:

> >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).
> 
> If it is a library, then it should be prepared to serve all possible users.

I don't against changing swiotlb library to make it usable for Xen.


> >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.
> 
> "Don't" in terms of "currently don't": Once x86-64 wants to support more
> than 46 physical address bits, it's not impossible that this would lead to
> CONFIG_HIGHMEM getting introduced there, and then it'll be helpful that the
> code is already prepared to deal with that case.

If you seriously think "adding highmem support to x86_64 would
happen", please take a look at:

http://lkml.org/lkml/2008/12/16/306


> After all, the code portion in question ought to compile to nothing if
> !CONFIG_HIGHMEM.

Adding such complication to the generic swiotlb code seriously hurts
its readability and maintainability. X86_64 and IA64 should not surfer
such damage.
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