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>

1 files changed, 4 insertions, 5 deletions

diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index c6ccbe7e81ad..9e2b952f810a 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -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");