[Xen-devel] [PATCH] x86: Fix build following c/s 623c720f "x86: use CLFLUSHOPT when available"

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

CentOS 7 gets into trouble when compiling Xen citing:

  flushtlb.c: Assembler messages:
  flushtlb.c:149: Error: value of 256 too large for field of 1 bytes at 1

The line number is wrong, and the error message not helpful.  It turns out
that the intermediate generated assembly was

  # 139 "arch/x86/flushtlb.c" 1
      661:
      rex clflush (%r15)
  662:
  .pushsection .altinstructions,"a"

and it was having trouble combining the explicit REX prefix with the REX.B
required for the use of %r15.

Follow what Linux does and use a redundant %ds prefix instead, for a final
generated instruction of `3e 41 0f ae 3f`

While modifying this line, fix the indentation which was out by one space.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/flushtlb.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/flushtlb.c b/xen/arch/x86/flushtlb.c
index 90a004f..727434e 100644
--- a/xen/arch/x86/flushtlb.c
+++ b/xen/arch/x86/flushtlb.c
@@ -141,10 +141,10 @@ void flush_area_local(const void *va, unsigned int flags)
         {
             alternative(ASM_NOP3, "sfence", X86_FEATURE_CLFLUSHOPT);
             for ( i = 0; i < sz; i += c->x86_clflush_size )
-                 alternative_input("rex clflush %0",
-                                   "data16 clflush %0",
-                                   X86_FEATURE_CLFLUSHOPT,
-                                   "m" (((const char *)va)[i]));
+                alternative_input(".byte 0x3e; clflush %0", /* %ds override. */
+                                  "data16 clflush %0",      /* clflushopt.   */
+                                  X86_FEATURE_CLFLUSHOPT,
+                                  "m" (((const char *)va)[i]));
         }
         else
         {
-- 
2.1.4


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