[Xen-changelog] [xen-unstable] x86/time: fix scale_delta() inline assembly

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Jan Beulich <jbeulich@xxxxxxxx>
# Date 1353946839 -3600
# Node ID 16bf7f3069a77c8a15b785cfdb5f2f634661d7fc
# Parent  0049de3827bcfe3488dc6c7b7a60327a562aee5c
x86/time: fix scale_delta() inline assembly

The way it was coded, it clobbered %rdx without telling the compiler.
This generally didn't cause any problems except when there are two back
to back invocations (as in plt_overflow()), as in that case the
compiler may validly assume that it can re-use for the second instance
the value loaded into %rdx before the first one.

Once at it, also properly relax the second operand of "mul" (there's no
need for it to be in %rdx, or a register at all), and switch away from
using explicit register names in the instruction operands.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Keir Fraser <keir@xxxxxxx>
---


diff -r 0049de3827bc -r 16bf7f3069a7 xen/arch/x86/time.c
--- a/xen/arch/x86/time.c       Fri Nov 23 11:06:15 2012 +0000
+++ b/xen/arch/x86/time.c       Mon Nov 26 17:20:39 2012 +0100
@@ -127,8 +127,9 @@ static inline u64 scale_delta(u64 delta,
         delta <<= scale->shift;
 
     asm (
-        "mul %%rdx ; shrd $32,%%rdx,%%rax"
-        : "=a" (product) : "0" (delta), "d" ((u64)scale->mul_frac) );
+        "mul %2 ; shrd $32,%1,%0"
+        : "=a" (product), "=d" (delta)
+        : "rm" (delta), "0" ((u64)scale->mul_frac) );
 
     return product;
 }

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