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

[David Sanders]

On Sunday 31 August 2008 16:03, David Sanders wrote:
> On Sunday 31 August 2008 14:47, Linus Torvalds wrote:
> > On Sun, 31 Aug 2008, David Sanders wrote:
> > > I recently discovered that x86 kernels won't boot under Virtual PC.
> >
> > What CPU does Virtual PC emulate? As far as Wikipedia is concerned (not
> > that I'd take it on complete faith) it emulates a 32-bit Intel Pentium
> > II.
> >
> > And that commit makes the kernel use the "P6 nops" for such hardware.
> > Maybe Virtual PC doesn't support the newer intel nop things?
> >
> > Intel docs say that it should be available on any intel CPU that has
> > CPUID.01H.EAX[11:8] = 0110B or 1111B. That's the "family ID", and Pentium
> > II should have a family ID of 6 (ie that 0110B case).
> >
> > So it sounds like a Virtual PC bug, but I dunno. And maybe we should just
> > use the legcay nops for anything that isn't modern (ie P4+ or Core)?
> >
> > 		Linus
>
> Virtual PC does not emulate a processor like it does with the motherboard,
> video, NIC ,etc.  What you see in the virtual machine is the actual
> processor, except that Virtual PC looks at all of your instructions and
> modifies ring 0 code and the like.  It may be that Virtual PC was not
> designed with an awareness of these nops that the commit added.
>
> I would suggest an configuration option to select legacy-nops or newer-nops
> and a kernel boot-time parameter so it can be disabled to allow
> installation of a distribution for example.
>
> I would be happy to submit such a patch if you agree (or I'll try to
> anyway).

OK, below is my patch.  The problem is I just built with it and the boot 
problem still exists.  Either the patch is flawed or git-bisect gave me the 
wrong commit.  Any help would be appreciated.
David

Signed-off-by: David Sanders <linux@xxxxxxxxxxxxxx>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index ed92864..a38f362 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -445,6 +445,13 @@ config PARAVIRT_DEBUG
          Enable to debug paravirt_ops internals.  Specifically, BUG if
 	 a paravirt_op is missing when it is called.
 
+config INTEL_NOPS
+	bool "Use legacy x86 NOPS"
+	depends on X86_32
+	help
+	This option tells kernel to use legacy x86 nops
+
+
 config MEMTEST
 	bool "Memtest"
 	help
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 2763cb3..10713e4 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -58,6 +58,16 @@ 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)
 
@@ -167,12 +177,17 @@ const unsigned char *const *find_nop_table(void)
 	const unsigned char *const *noptable = intel_nops;
 	int i;
 
-	for (i = 0; noptypes[i].cpuid >= 0; i++) {
-		if (boot_cpu_has(noptypes[i].cpuid)) {
-			noptable = noptypes[i].noptable;
-			break;
+	if (!intel_legacy_nops) {
+#ifndef CONFIG_INTEL_NOPS
+		for (i = 0; noptypes[i].cpuid >= 0; i++) {
+			if (boot_cpu_has(noptypes[i].cpuid)) {
+				noptable = noptypes[i].noptable;
+				break;
+			}
 		}
+#endif /* CONFIG_INTEL_NOPS */
 	}
+
 	return noptable;
 }
--
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