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

[Jeremy Fitzhardinge]

FUJITA Tomonori wrote:
> On Tue, 16 Dec 2008 21:35:13 +0100
> Ingo Molnar <mingo@xxxxxxx> wrote:
>
>   
> > * Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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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