Re: asm (lidt) question

[Davide Libenzi <davidel@xmailserver.org>]

On Thu, 17 Jul 2003, Davide Libenzi wrote:

> On Thu, 17 Jul 2003, Randy.Dunlap wrote:
>
> >
> > In arch/i386/kernel, inline asm for loading IDT (lidt) is used a few
> > times, but with slightly different constraints and output/input
> > definitions.  Are these OK, equivalent, or what?
> >
> > [rddunlap@dragon kernel]$ findc lidt
> > ./cpu/common.c:484: __asm__ __volatile__("lidt %0": "=m" (idt_descr));
> > ./traps.c:783:	__asm__ __volatile__("lidt %0": "=m" (idt_descr));
> >
> > vs.
> >
> > ./reboot.c:186:	__asm__ __volatile__ ("lidt %0" : : "m" (real_mode_idt));
> > ./reboot.c:261:	__asm__ __volatile__("lidt %0": :"m" (no_idt));
> > ./suspend.c:95:	asm volatile ("lidt %0" :: "m" (saved_context.idt_limit));
>
> I'd have said no looking at the syntax (input/output), but they indeed
> generate the same code (just checked).

Randy, I'd say that this :

__asm__ __volatile__("lidt %0": "=m" (var));

is better then :

__asm__ __volatile__("lidt %0": :"m" (var));

IMHO, since "var" is really an output parameter.



- Davide