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SubjectRe: [PATCH v2 03/22] ARM: use late patch framework for phys-virt patching
On Sun, 12 Aug 2012, Cyril Chemparathy wrote:

> On 08/11/12 23:03, Nicolas Pitre wrote:
> > On Fri, 10 Aug 2012, Cyril Chemparathy wrote:
> >
> > > -extern unsigned long __pv_phys_offset;
> > > -#define PHYS_OFFSET __pv_phys_offset
> [...]
> > > +#define PHYS_OFFSET __virt_to_phys(PAGE_OFFSET)
> >
> > What was wrong with the former PHYS_OFFSET = __pv_phys_offset ?
> >
> > If you really want to have it optimized at run time, you could simply
> > use your new stub to patch a mov instruction instead of going through
> > __virt_to_phys which uses and add on top of a constant.
> >
>
> The intent was to optimize out the load(s) on references to PHYS_OFFSET, but
> is it worth it? If so, we could go with a patched mov (or two) with the
> necessary endian fixups. If not, we could revert to __pv_phys_offset loads as
> before.

If you want to do better than the load, then you'd better go all the way
with the patched move.

> > You also should remove the MODULE_ARCH_VERMAGIC_P2V definitions now that
> > the corresponding code is no longer there.
> >
>
> Hmm...
>
> "rt-patch" needs to be in vermagic to prevent modules built against the new
> code from being loaded on older kernels that used the traditional patch code.

Right.

> "p2v" needs to be in there as well, because it should be possible to build
> without PATCH_PHYS_VIRT, but with RUNTIME_PATCH as and when there are other
> users for this.

That doesn't matter if there are other users. As soon as there is a
.init.runtime_patch_table that needs to be processed then "rt-patch"
flags it. Whether this is used for PATCH_PHYS_VIRT or other purposes is
irrelevant.

However there isn't any pv_table anymore, so "p2v" should go as there is
no more code to process those tables if they're ever encountered.


Nicolas


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