[PATCH v2] x86, boot: Fix word-size assumptions in has_eflag ()inline asm

[David Woodhouse]

Commit dd78b97367bd575918204cc89107c1479d3fc1a7 ("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.

Signed-off-by: David Woodhouse <David.Woodhouse@xxxxxxxxx>
---

Let me know if you'd rather have this as an incremental patch. I would
have preferred checking for BITS_PER_LONG==64 rather than __x86_64__
but it seems we set that to 64 even when building the 16-bit code.

 arch/x86/boot/cpuflags.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/arch/x86/boot/cpuflags.c b/arch/x86/boot/cpuflags.c
index a9fcb7c..431fa5f 100644
--- a/arch/x86/boot/cpuflags.c
+++ b/arch/x86/boot/cpuflags.c
@@ -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));
 
-- 
1.8.5.3



-- 
dwmw2

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