Commit ed68d7e9 authored Nov 19, 2016 by Andy Lutomirski Committed by Ingo Molnar Nov 21, 2016

x86/boot: Fail the boot if !M486 and CPUID is missing



Linux will have all kinds of sporadic problems on systems that don't
have the CPUID instruction unless CONFIG_M486=y.  In particular,
sync_core() will explode.

I believe that these kernels had a better chance of working before
commit 05fb3c19 ("x86/boot: Initialize FPU and X86_FEATURE_ALWAYS
even if we don't have CPUID").  That commit inadvertently fixed a
serious bug: we used to fail to detect the FPU if CPUID wasn't
present.  Because we also used to forget to set X86_FEATURE_ALWAYS, we
end up with no cpu feature bits set at all.  This meant that
alternative patching didn't do anything and, if paravirt was disabled,
we could plausibly finish the entire boot process without calling
sync_core().

Rather than trying to work around these issues, just have the kernel
fail loudly if it's running on a CPUID-less 486, doesn't have CPUID,
and doesn't have CONFIG_M486 set.

Reported-by: Matthew Whitehead <tedheadster@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/70eac6639f23df8be5fe03fa1984aedd5d40077a.1479598603.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent fc0e81b2

arch/x86/boot/cpu.c

+6 −0

Original line number	Original line	Diff line number	Diff line
	@@ -87,6 +87,12 @@ int validate_cpu(void)
	return -1;		return -1;
	}		}

			if (CONFIG_X86_MINIMUM_CPU_FAMILY <= 4 && !IS_ENABLED(CONFIG_M486) &&
			!has_eflag(X86_EFLAGS_ID)) {
			printf("This kernel requires a CPU with the CPUID instruction. Build with CONFIG_M486=y to run on this CPU.\n");
			return -1;
			}

	if (err_flags) {		if (err_flags) {
	puts("This kernel requires the following features "		puts("This kernel requires the following features "
	"not present on the CPU:\n");		"not present on the CPU:\n");

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