From owner-freebsd-arch  Mon Nov 12  4:10:31 2001
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Date: Mon, 12 Nov 2001 23:09:06 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Matthew Dillon <dillon@apollo.backplane.com>
Cc: Peter Wemm <peter@wemm.org>, Robert Watson <rwatson@FreeBSD.ORG>,
	<freebsd-arch@FreeBSD.ORG>
Subject: Re: cur{thread/proc}, or not. 
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On Mon, 12 Nov 2001, Matthew Dillon wrote:

>     If we stopped trying to be fancy with interrupt scheduling and went
>     back to the BSDI methodology the kernel code could assume that
>     %fs doesn't change out from under it and we could *GREATLY*

Strictly, that the GDT entry for %fs doesn't change.  We could safely
assume this already for the !SMP case.

>     simplify the __PCPU_GET() code to something like this:
>
> static __inline
> struct globaldata *
> __globaldata(void)
> {
>         struct globaldata *gd;
>
>         __asm("movl %%fs,%0" : "=r" (gd));
>         return(gd);
> }
>
> #define __PCPU_GET(name)        (__globaldata()->name)
>
>     Which would allow GCC to generate somewhat better code output
>     (about 1K less code in the text segment as well) as well as
>     allow the per-cpu variables to be accessed more normally without
>     having to macros to GET and SET them.

This is essentially a slightly pessimized version of the RELENG_4
code for the SMP case (RELENG_4 avoids going through the pointer in
for most per-cpu global accesses).

It also helps to declare __globaldata() as __pure2 so that gcc can
tell that it always returns the same value.  It doesn't quite always
return the same value, but I can't think of any cases where a cached
value would remain valid long enough to cause problems.

Bruce


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