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x86: allow "=rm" in native_save_fl() 

This is a partial revert of f1f029c7bf.

"=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 f1f029c7bf was
apparently using a broken x86 simulator.

[ Impact: performance ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Gabe Black <spamforgabe@umich.edu>
This commit is contained in:
H. Peter Anvin 2009-08-25 16:47:16 -07:00
parent e8a2eb47e6
commit ab94fcf528
1 changed files with 4 additions and 5 deletions
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9
arch/x86/include/asm/irqflags.h
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@ -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");