Re: [PATCH 1/10] Cr4 is valid on some 486s

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Linus Torvalds wrote:

On Mon, 14 Nov 2005, Gerd Knorr wrote:
Throwing another patch into the discussion ;)

Ouch, this one is really ugly.

I somehow expected that answer, it took me quite some time to figure what the patch does. It certainly needs at least a number of cleanups before I'd consider it mergable. The alternative() macro is much easier to read.

If you want to go this way, then you should instead add an X86_FEATURE_SMP that gets cleared on UP and on SMP with just one core (and detect when CPU hotplug ain't gonna happen ;), and then do

Well, the "no hotplug" probably is exactly the reason why the patch doesn't use the existing alternatives mechanism, it's a boot-time one-way ticket. The xenified linux kernel actually switches both ways at runtime if you plug in/out a second virtual CPU.

	#ifdef CONFIG_SMP
	#define smp_alternative(x,y) alternative(x,y,X86_FEATURE_SMP)
	#else
	#define smp_alternative(x,y) asm(x)
	#endif

I don't like the idea very much. That covers only 50% of what the patch does, you can patch SMP => UP but not the other way around. Doesn't matter much on real hardware, but for virtual it is quite useful.

or something similar, instead of creating a totally new infrastructure to do the thing that "alternative()" already does.

Yep, extending alternatives is probably better than duplicating the code. Maybe having some alternative_smp() macro which places both code versions into the .altinstr_replacement table? If that sounds ok I'll try to come up with a experimental patch. If not: other ideas are welcome.

cheers,

  Gerd

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