Date: Mon, 15 Jan 2001 13:17:21 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: "Justin T. Gibbs" <gibbs@scsiguy.com> Cc: Peter Wemm <peter@netplex.com.au>, Poul-Henning Kamp <phk@critter.freebsd.dk>, Wilko Bulte <wkb@freebie.demon.nl>, cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org, Bruce Evans <bde@zeta.org.au> Subject: Re: cvs commit: src/sys/i386/conf GENERIC Message-ID: <XFMail.010115131721.jhb@FreeBSD.org> In-Reply-To: <200101152012.f0FKCns56756@aslan.scsiguy.com>
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On 15-Jan-01 Justin T. Gibbs wrote:
>>So are you ready to write the code in trap() to handle an illegal instruction
>>fault in userland that decodes and executes all variants of cmpxchg? The new
>>threading code in libc will be using atomic_cmpset() from userland, which is
>>going to be the main hurdle to get over.
>
> This is the wrong way to handle it. Have atomic_cmpset() perform a fixup
> of the calling code on first entry and the result will be code as optimized
> as possible for the processor type the code is running on. If the user
> decides to write their own code that uses cmpxchg, they get what they
> deserve, but the primitives should not require a *fault* to work correctly.
*sigh*
Go look at the 386 version of atomic_cmpset in atomic.h:
#if defined(I386_CPU)
static __inline int
atomic_cmpset_int(volatile u_int *dst, u_int exp, u_int src)
{
int res = exp;
__asm __volatile(
" pushfl ; "
" cli ; "
" cmpl %1,%3 ; "
" jne 1f ; "
" movl %2,%3 ; "
"1: "
" sete %%al; "
" movzbl %%al,%0 ; "
" popfl ; "
"# atomic_cmpset_int"
: "=a" (res) /* 0 (result) */
: "0" (exp), /* 1 */
"r" (src), /* 2 */
"m" (*(dst)) /* 3 */
: "memory");
return (res);
}
See those 'cli' and 'popfl' instrucitons? Those are _privileged_. Userland
can't disable/enable interrupts, so we have to trap into the kernel to do this
no matter what. If you want to patch the code to do a syscall instead of a
cmpxchg instruction, fine. However, emulating atomic_cmpset in userland on a
386 requires a trap into the kernel. Please assume for at least 1 minute that
the SMPng guys are not complete bumbling idiots and that we may have actually
thought about this for at least 5 minutes.
--
John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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