linux-kernel - Andi, you broke my laptop :-)

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Message-Id: <20070509125616.cde56b8e.zaitcev@redhat.com>
Date:	Wed, 9 May 2007 12:56:16 -0700
From:	Pete Zaitcev <zaitcev@...hat.com>
To:	<ak@...e.de>
Cc:	zaitcev@...hat.com, linux-kernel@...r.kernel.org,
	<joerg.roedel@....com>
Subject: Andi, you broke my laptop :-)

Hi, Andi:

The attached patch (actually, git show output) makes my Dell 1501 to hang
on boot. Sorry, I have no clue why... The culprit is found with git bisect.
But yes, it's an AMD MK-36. I use an x86_64 kernel. It is 100% reproducible.

Cheers,
-- Pete

commit c5bcb5635a03da3158f121ae20ccbbf72b4fc62a
Author: Andi Kleen <ak@...e.de>
Date:   Wed May 2 19:27:21 2007 +0200

    [PATCH] x86: Use RDTSCP for synchronous get_cycles if possible
    
    RDTSCP is already synchronous and doesn't need an explicit CPUID.
    This is a little faster and more importantly avoids VMEXITs on Hypervisors.
    
    Original patch from Joerg Roedel, but reworked by AK
    Also includes miscompilation fix by Eric Biederman
    
    Cc: "Joerg Roedel" <joerg.roedel@....com>
    
    Signed-off-by: Andi Kleen <ak@...e.de>

diff --git a/include/asm-i386/tsc.h b/include/asm-i386/tsc.h
index 0181f9d..3f3c1fa 100644
--- a/include/asm-i386/tsc.h
+++ b/include/asm-i386/tsc.h
@@ -38,6 +38,15 @@ static __always_inline cycles_t get_cycles_sync(void)
 	unsigned eax;
 
 	/*
+  	 * Use RDTSCP if possible; it is guaranteed to be synchronous
+ 	 * and doesn't cause a VMEXIT on Hypervisors
+	 */
+	alternative_io(ASM_NOP3, ".byte 0x0f,0x01,0xf9", X86_FEATURE_RDTSCP,
+			 	 "=A" (ret), "0" (0ULL) : "ecx", "memory");
+	if (ret)
+		return ret;
+
+	/*
 	 * Don't do an additional sync on CPUs where we know
 	 * RDTSC is already synchronous:
 	 */
-
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