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From: Borislav Petkov <bp@alien8.de>
To: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Mario Gzuk <mariogzuk@technikz.de>
Subject: Re: [PATCH 8/8] x86, cleanups: Simplify sync_core() in the case of no CPUID
Date: Thu, 29 Nov 2012 22:31:51 +0100
Message-ID: <20121129213151.GI30789@liondog.tnic> (raw)
In-Reply-To: <50B7D180.9090605@linux.intel.com>

On Thu, Nov 29, 2012 at 01:20:00PM -0800, H. Peter Anvin wrote:
> On 11/29/2012 01:18 PM, Borislav Petkov wrote:
> > On Thu, Nov 29, 2012 at 01:06:20PM -0800, H. Peter Anvin wrote:
> >> It doesn't matter in that context, as the surrounding MSR references
> >> have barriers, but what I'm refering to is the "memory" barrier.
> > 
> > Ok, but the only difference between the two versions is this line:
> > 
> >         movl    %esi, %ecx      # tmp144, ecx
> > 
> > coming from the cpuid_eax() function. So the memory barrier is the same
> > and in the right place in both cases.
> > 
> 
> In the case of that one call site, yes, because the MSR references
> include the barrier.  Other sites, current or future, may not have the
> same property.

Sorry, but I think we're misunderstanding each other in some way, so let
me restart. Here's the version I'm suggesting:

static inline void sync_core(void)
{
	int tmp;

#ifdef CONFIG_M486
	/*
	 * Do a CPUID if available, otherwise do a jump.  The jump
	 * can conveniently enough be the jump around CPUID.
	 */
	asm volatile("cmpl %2,%1\n\t"
		     "jl 1f\n\t"
		     "cpuid\n"
		     "1:"
		     : "=a" (tmp)
		     : "rm" (boot_cpu_data.cpuid_level), "ri" (0), "0" (1)
		     : "ebx", "ecx", "edx", "memory");
#else
	/*
	 * CPUID is a barrier to speculative execution. Prefetched instructions
	 * are automatically invalidated when modified.
	 */
	tmp = cpuid_eax(1);

	/*
	asm volatile("cpuid" : "=a" (tmp) : "0" (1)
		     : "ebx", "ecx", "edx", "memory");
	*/
#endif
}

with the last asm volatile("cpuid"...) commented out. The only
non-trivial difference between the two is the zeroing out of %ecx when
looking at the resulting asm.

Now, cpuid_eax is actually native_cpuid() which has the memory barrier
character by having an "asm volatile" in there too and it too clobbers
memory.

Are you saying that there's a semantic difference between the naked
"asm volatile" and the compiler inlining a couple of inline functions
resulting in the same "asm volatile" memory barrier for it?

Hmm, strange.

-- 
Regards/Gruss,
    Boris.

  reply index

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-11-28 19:50 [PATCH 0/8] RFC: Remove 386 support H. Peter Anvin
2012-11-28 19:50 ` [PATCH 1/8] x86, 386 removal: Remove CONFIG_M386 from Kconfig H. Peter Anvin
2012-12-01  0:34   ` [tip:x86/nuke386] " tip-bot for H. Peter Anvin
2012-11-28 19:50 ` [PATCH 2/8] x86, 386 removal: Remove CONFIG_CMPXCHG H. Peter Anvin
2012-12-01  0:35   ` [tip:x86/nuke386] " tip-bot for H. Peter Anvin
2012-11-28 19:50 ` [PATCH 3/8] x86, 386 removal: Remove CONFIG_XADD H. Peter Anvin
2012-11-28 20:30   ` Borislav Petkov
2012-12-01  0:36   ` [tip:x86/nuke386] " tip-bot for H. Peter Anvin
2012-11-28 19:50 ` [PATCH 4/8] x86, 386 removal: Remove CONFIG_BSWAP H. Peter Anvin
2012-12-01  0:37   ` [tip:x86/nuke386] " tip-bot for H. Peter Anvin
2012-11-28 19:50 ` [PATCH 5/8] x86, 386 removal: Remove CONFIG_INVLPG H. Peter Anvin
2012-12-01  0:38   ` [tip:x86/nuke386] " tip-bot for H. Peter Anvin
2012-11-28 19:50 ` [PATCH 6/8] x86, 386 removal: Remove CONFIG_X86_WP_WORKS_OK H. Peter Anvin
2012-11-28 20:52   ` Alan Cox
2012-11-28 23:11     ` H. Peter Anvin
2012-12-01  0:42     ` [tip:x86/nuke386] x86, 386 removal: Document Nx586 as a 386 and thus unsupported tip-bot for H. Peter Anvin
2012-12-01  0:39   ` [tip:x86/nuke386] x86, 386 removal: Remove CONFIG_X86_WP_WORKS_OK tip-bot for H. Peter Anvin
2012-11-28 19:50 ` [PATCH 7/8] x86, 386 removal: Remove CONFIG_X86_POPAD_OK H. Peter Anvin
2012-12-01  0:40   ` [tip:x86/nuke386] " tip-bot for H. Peter Anvin
2012-11-28 19:50 ` [PATCH 8/8] x86, cleanups: Simplify sync_core() in the case of no CPUID H. Peter Anvin
2012-11-28 20:41   ` Borislav Petkov
2012-11-29  0:14     ` H. Peter Anvin
2012-11-29  9:13       ` Borislav Petkov
2012-11-29 21:06         ` H. Peter Anvin
2012-11-29 21:18           ` Borislav Petkov
2012-11-29 21:20             ` H. Peter Anvin
2012-11-29 21:31               ` Borislav Petkov [this message]
2012-11-29 21:24     ` H. Peter Anvin
2012-11-29 21:34       ` Borislav Petkov
2012-11-30 17:01       ` Linus Torvalds
2012-11-30 17:28         ` Borislav Petkov
2012-11-30 18:03         ` H. Peter Anvin
2012-11-30 18:10           ` Borislav Petkov
2012-11-30 18:40             ` H. Peter Anvin
2012-12-01  0:41   ` [tip:x86/nuke386] " tip-bot for H. Peter Anvin

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