linux-kernel - Re: [BUG] x86 kenel won't boot under Virtual PC

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Message-Id: <200809051138.20615.linux@sandersweb.net>
Date:	Fri, 5 Sep 2008 11:38:19 -0400
From:	David Sanders <linux@...dersweb.net>
To:	linux-kernel@...r.kernel.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi-suse@...stfloor.org>
Subject: Re: [BUG] x86 kenel won't boot under Virtual PC

On Sun, 31 Aug 2008, David Sanders wrote:
> I recently discovered that x86 kernels won't boot under Virtual PC.On Sun, 
31 Aug 2008, David Sanders wrote:

On Sunday 31 August 2008 15:39, Linus Torvalds wrote:
> Well, it could be that Virtual PC raises a #UD exception in the virtual
> machine. In user space, that would just cause the kernel to kill the poor
> innocent victim. But when the kernel gets a #UD exception on what it
> expects to be a nop, it just won't work.

Sorry about the confusion. I was just learning git and applied a couple 
patches wrong that lead me to believe I had not found the answer, when in 
fact I had.  The problem is the multibyte NOP instructions introduced into 
the kernel in 2007.  For 2.6.24-rc1 reverting the patch (32c464f5) corrects 
the problem.  But then of course we cannot use these instructions as we would 
like.  I propose the following patch which adds a boot-time parameter to not 
use these multibyte NOPs in systems that are unprepared to deal with them.  
The patch disables a optimization used in one place by commenting out some 
lines in nop.h.  Please comment.

    x86: Fix to multibyte NOPs breaking Virtual PC (win)

    To be applied to linux-2.6.27-rc5.

    Signed-off-by: David Sanders <linux@...dersweb.net>

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 2763cb3..6a63e78 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -58,6 +58,15 @@ static int __init setup_noreplace_paravirt(char *str)
 __setup("noreplace-paravirt", setup_noreplace_paravirt);
 #endif
 
+static int intel_legacy_nops = 0;
+
+static int __init setup_intel_legacy_nops(char *str)
+{
+	intel_legacy_nops = 1;
+	return 1;
+}
+__setup("intel-legacy-nops", setup_intel_legacy_nops);
+
 #define DPRINTK(fmt, args...) if (debug_alternative) \
 	printk(KERN_DEBUG fmt, args)
 
@@ -165,13 +174,14 @@ static const struct nop {
 const unsigned char *const *find_nop_table(void)
 {
 	const unsigned char *const *noptable = intel_nops;
-	int i;
+	int i = 0;
 
-	for (i = 0; noptypes[i].cpuid >= 0; i++) {
+	while (noptypes[i].cpuid >= 0 && !intel_legacy_nops) {
 		if (boot_cpu_has(noptypes[i].cpuid)) {
 			noptable = noptypes[i].noptable;
 			break;
 		}
+		i++;
 	}
 	return noptable;
 }
diff --git a/include/asm-x86/nops.h b/include/asm-x86/nops.h
index ad0bedd..a6f0571 100644
--- a/include/asm-x86/nops.h
+++ b/include/asm-x86/nops.h
@@ -84,7 +84,7 @@
 #define ASM_NOP6 K7_NOP6
 #define ASM_NOP7 K7_NOP7
 #define ASM_NOP8 K7_NOP8
-#elif defined(CONFIG_X86_P6_NOP)
+/* #elif defined(CONFIG_X86_P6_NOP)
 #define ASM_NOP1 P6_NOP1
 #define ASM_NOP2 P6_NOP2
 #define ASM_NOP3 P6_NOP3
@@ -93,6 +93,7 @@
 #define ASM_NOP6 P6_NOP6
 #define ASM_NOP7 P6_NOP7
 #define ASM_NOP8 P6_NOP8
+*/
 #elif defined(CONFIG_X86_64)
 #define ASM_NOP1 K8_NOP1
 #define ASM_NOP2 K8_NOP2
--
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