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SubjectRe: [BUG] x86 kenel won't boot under Virtual PC
Date
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@sandersweb.net>

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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