Date: Mon, 12 Nov 2001 23:09:06 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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.
Message-ID: <20011112221522.E36389-100000@delplex.bde.org>
In-Reply-To: <200111121009.fACA9SI75024@apollo.backplane.com>
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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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