Date: Sat, 15 Sep 2001 12:06:20 -0700 (PDT) From: John Polstra <jdp@polstra.com> To: stable@freebsd.org Cc: dres@earth.serd.org Subject: Re: sys/i386/i386/identcpu.c bugfix Message-ID: <200109151906.f8FJ6Kk74842@vashon.polstra.com> In-Reply-To: <20010905160610.U1222-100000@earth.serd.org> References: <20010905160610.U1222-100000@earth.serd.org>
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In article <20010905160610.U1222-100000@earth.serd.org>,
Stefan Keller <dres@earth.serd.org> wrote:
>
> the do_cpuid() inline asm routine in identcpu.c has an incomplete
> description of clobbered registers, which shows up when the kernel is
> compiled with higher optimisations turned on.
> The fix:
>
> --- sys/i386/i386/identcpu.c.org Mon Sep 3 23:44:25 2001
> +++ sys/i386/i386/identcpu.c Wed Sep 5 03:49:16 2001
> @@ -121,7 +121,7 @@
> "movl %%edx, 12(%2);"
> : "=a" (ax)
> : "0" (ax), "S" (p)
> - : "bx", "cx", "dx"
> + : "bx", "cx", "dx", "cc", "memory"
> );
> }
>
> Not specifying "memory" causes gcc to cache the contents of
> the memory pointed to by p across do_cpuid() calls.
> On my system (Athlon processor) this only showed up on the
> CPU name; everything else was ok.
Nice catch! I really encourage anybody who can do so to check our
asm statements. I'm sure there are a lot more of them with incorrect
constraints, and they are the main reason we have problems at -O2 and
higher. These problems often get blamed on compiler bugs, but I don't
believe it. Both Linux and BSD/OS use -O2 without problems.
About your patch: I made a different version which I think is slightly
better, because it specifies the memory clobbers more specifically.
It's simpler, too. I checked the compiler's assembly language output,
and it looks like it's doing the right thing. Could you please give
it a try? Note, I removed the clobber of "cc" because the Intel
book says that no flags are affected by the CPUID instruction.
This is relative to today's -stable:
Index: identcpu.c
===================================================================
RCS file: /home/ncvs/src/sys/i386/i386/identcpu.c,v
retrieving revision 1.80.2.6
diff -c -r1.80.2.6 identcpu.c
*** identcpu.c 2001/07/19 09:12:07 1.80.2.6
--- identcpu.c 2001/09/15 19:04:23
***************
*** 114,127 ****
do_cpuid(u_int ax, u_int *p)
{
__asm __volatile(
! ".byte 0x0f, 0xa2;"
! "movl %%eax, (%2);"
! "movl %%ebx, 4(%2);"
! "movl %%ecx, 8(%2);"
! "movl %%edx, 12(%2);"
! : "=a" (ax)
: "0" (ax), "S" (p)
- : "bx", "cx", "dx"
);
}
--- 114,122 ----
do_cpuid(u_int ax, u_int *p)
{
__asm __volatile(
! ".byte 0x0f, 0xa2"
! : "=a" (p[0]), "=b" (p[1]), "=c" (p[2]), "=d" (p[3])
: "0" (ax), "S" (p)
);
}
John
--
John Polstra jdp@polstra.com
John D. Polstra & Co., Inc. Seattle, Washington USA
"Disappointment is a good sign of basic intelligence." -- Chögyam Trungpa
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