Commit 5fbbc25a authored Jan 30, 2014 by David Woodhouse Committed by H. Peter Anvin Jan 30, 2014

x86, boot: Fix word-size assumptions in has_eflag() inline asm



Commit dd78b973 ("x86, boot: Move CPU
flags out of cpucheck") introduced ambiguous inline asm in the
has_eflag() function. In 16-bit mode want the instruction to be
'pushfl', but we just say 'pushf' and hope the compiler does what we
wanted.

When building with 'clang -m16', it won't, because clang doesn't use
the horrid '.code16gcc' hack that even 'gcc -m16' uses internally.

Say what we mean and don't make the compiler make assumptions.

[ hpa: ideally we would be able to use the gcc %zN construct here, but
  that is broken for 64-bit integers in gcc < 4.5.

  The code with plain "pushf/popf" is fine for 32- or 64-bit mode, but
  not for 16-bit mode; in 16-bit mode those are 16-bit instructions in
  .code16 mode, and 32-bit instructions in .code16gcc mode. ]

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Link: http://lkml.kernel.org/r/1391079628.26079.82.camel@shinybook.infradead.org


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

parent 4064e0ea

arch/x86/boot/cpuflags.c

+20 −5

Original line number	Diff line number	Diff line
		@@ -28,20 +28,35 @@ static int has_fpu(void)
		return fsw == 0 && (fcw & 0x103f) == 0x003f;
		}

		/*
		* For building the 16-bit code we want to explicitly specify 32-bit
		* push/pop operations, rather than just saying 'pushf' or 'popf' and
		* letting the compiler choose. But this is also included from the
		* compressed/ directory where it may be 64-bit code, and thus needs
		* to be 'pushfq' or 'popfq' in that case.
		*/
		#ifdef __x86_64__
		#define PUSHF "pushfq"
		#define POPF "popfq"
		#else
		#define PUSHF "pushfl"
		#define POPF "popfl"
		#endif

		int has_eflag(unsigned long mask)
		{
		unsigned long f0, f1;

		asm volatile("pushf \n\t"
		"pushf \n\t"
		asm volatile(PUSHF " \n\t"
		PUSHF " \n\t"
		"pop %0 \n\t"
		"mov %0,%1 \n\t"
		"xor %2,%1 \n\t"
		"push %1 \n\t"
		"popf \n\t"
		"pushf \n\t"
		POPF " \n\t"
		PUSHF " \n\t"
		"pop %1 \n\t"
		"popf"
		POPF
		: "=&r" (f0), "=&r" (f1)
		: "ri" (mask));

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