Messages in this thread Patch in this message | | | From | David Sanders <> | Subject | Re: [BUG] x86 kenel won't boot under Virtual PC | Date | Fri, 5 Sep 2008 11:38:19 -0400 |
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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@sandersweb.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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