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

FUJITA Tomonori
2008-12-17T16:51:58Z

On Wed, 17 Dec 2008 08:47:47 +0000
"Jan Beulich" <jbeulich@novell.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.