linux-kernel - Re: [PATCH 8/8] x86, cleanups: Simplify sync

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Message-ID: <20121129091317.GA21084@liondog.tnic>
Date:	Thu, 29 Nov 2012 10:13:17 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mario Gzuk <mariogzuk@...hnikz.de>
Subject: Re: [PATCH 8/8] x86, cleanups: Simplify sync_core() in the case of
 no CPUID

On Wed, Nov 28, 2012 at 04:14:43PM -0800, H. Peter Anvin wrote:
> Wrong barrier semantics.

Let me try to understand what you mean by that :)

Now, your version's asm output looks like this:

----
	.loc 2 95 0
	movl	$139, %esi	#, tmp140
	xorl	%eax, %eax	# tmp141
	movl	%esi, %ecx	# tmp140,
	movl	%eax, %edx	# tmp141,
#APP
# 95 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/msr.h" 1
	wrmsr
# 0 "" 2
#NO_APP
	.loc 3 694 0
	movb	$1, %al	#,
#APP
# 694 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/processor.h" 1
	cpuid
# 0 "" 2
.LVL15:
#NO_APP
----

This is the sync_core() call from early_init_intel(). Now look like 139
remains in %ecx after the WRMSR and CPUID actually gets called with
RAX=1 and RCX=139 (btw, 139 is MSR_IA32_UCODE_REV). Even if this works
I'd say, we don't want to have any stray values in RCX when doing CPUID,
no?

Now here's the version with the change I proposed:

----
	.loc 2 95 0
	xorl	%esi, %esi	# tmp144
	movl	$139, %edi	#, tmp143
	movl	%edi, %ecx	# tmp143,
	movl	%esi, %eax	# tmp144,
	movl	%esi, %edx	# tmp144,
#APP
# 95 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/msr.h" 1
	wrmsr
# 0 "" 2
.LVL15:
#NO_APP
	.loc 3 199 0
	movb	$1, %al	#,
	movl	%esi, %ecx	# tmp144, ecx
#APP
# 199 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/processor.h" 1
	cpuid
# 0 "" 2
.LVL16:
#NO_APP
----

RCX gets correctly cleaned to 0 and *then* we call CPUID.

And the asm output is the same except that RCX gets correctly cleaned up
before calling CPUID.

So what am I missing?

Thanks.

-- 
Regards/Gruss,
    Boris.
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