linux-kernel - Re: [PATCH v2 03/22] ARM: use late patch framework for phys-virt patching

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Date:	Sun, 12 Aug 2012 23:32:44 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Cyril Chemparathy <cyril@...com>
cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <catalin.marinas@....com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>
Subject: Re: [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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