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Commit ab94fcf5 authored by H. Peter Anvin's avatar H. Peter Anvin
Browse files

x86: allow "=rm" in native_save_fl() 



This is a partial revert of f1f029c7.

"=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 f1f029c7 was
apparently using a broken x86 simulator.

[ Impact: performance ]

Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
Cc: Gabe Black <spamforgabe@umich.edu>
parent e8a2eb47

arch/x86/include/asm/irqflags.h

+4 −5
Original line number Diff line number Diff line
@@ -13,14 +13,13 @@ static inline unsigned long native_save_fl(void) 
	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");