x86: allow "=rm" in native_save_fl()
This is a partial revert of f1f029c7bfbf4ee1918b90a431ab823bed812504.
"=rm" is allowed in this context, because "pop" is explicitly defined
to adjust the stack pointer *before* it evaluates its effective
address, if it has one. Thus, we do end up writing to the correct
address even if we use an on-stack memory argument.
The original reporter for f1f029c7bfbf4ee1918b90a431ab823bed812504 was
apparently using a broken x86 simulator.
[ Impact: performance ]
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Gabe Black <spamforgabe@umich.edu>
diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index c6ccbe7..9e2b952 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -13,14 +13,13 @@
unsigned long flags;
/*
- * Note: this needs to be "=r" not "=rm", because we have the
- * stack offset from what gcc expects at the time the "pop" is
- * executed, and so a memory reference with respect to the stack
- * would end up using the wrong address.
+ * "=rm" is safe here, because "pop" adjusts the stack before
+ * it evaluates its effective address -- this is part of the
+ * documented behavior of the "pop" instruction.
*/
asm volatile("# __raw_save_flags\n\t"
"pushf ; pop %0"
- : "=r" (flags)
+ : "=rm" (flags)
: /* no input */
: "memory");