linux-kernel - Re: RE : Re: RE : Re: x86 setup code rewrite in C

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Message-ID: <469B3714.80105@zytor.com>
Date:	Mon, 16 Jul 2007 02:15:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Etienne Lorrain <etienne_lorrain@...oo.fr>
CC:	Chuck Ebbert <cebbert@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: RE : Re: RE : Re: x86 setup code rewrite in C - revised

Etienne Lorrain wrote:

> BUGS:	some implementations (including the original IBM PC) have a bug which
> 	  destroys BP
> 	the Trident TVGA8900CL (BIOS dated 1992/9/8) clears DS to 0000h when
> 	  scrolling in an SVGA mode (800x600 or higher)

"When scrolling in an SVGA mode", sounds to me like a bug when using
BIOS for text output in graphics mode.  We don't do that.

>>>>>>  Also, I do not know if "m" is right in here:
>>>>>> static inline u8 rdfs8(addr_t addr)
>>>>>> {
>>>>>> 	u8 v;
>>>>>> 	asm("movb %%fs:%1,%0" : "=r" (v) : "m" (*(u8 *)addr));
>>>>>> 	return v;
>>>>>> }
>>>> The "m" is correct right there.
>>>  strange, "g" would mean anything can go there - and this assembly instruction
>>> should accept every access modes.
>> Not with an %fs: prefix.  It would also allow the compiler to do a move
>> into a register "on its own", which would be disastrous, since it would
>> lack the prefix.  So "m" is correct.
> 
>   "mov %fs:(%ebx,%eax,4),%ecx" works for me.

That's an example on what "m" can generate.

"g" could produce stuff like:

	mov %fs:$1234,%ecx
	mov %fs:%eax,%ecx

	-hpa
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