[Xen-changelog] [xen master] x86: fix build following c/s 623c720f "x86: use CLFLUSHOPT when available"

[patchbot@xxxxxxx]

commit d4ab00d292e7b1f909097e733d9e46e16e84fc38
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Feb 12 10:32:52 2016 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Feb 12 10:32:52 2016 +0100

    x86: fix build following c/s 623c720f "x86: use CLFLUSHOPT when available"
    
    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>
    Tested-by: Doug Goldstein <cardoe@xxxxxxxxxx>
    
    Introduce and use NOP_DS_PREFIX.
    
    Also the above description of the issue is slightly wrong: We're not
    suffering from some gas versions not being able to combine multiple REX
    prefixes, but from the replacement instruction, when requiring a REX
    prefix in order to express the memory operand, becoming one byte longer
    than the original one, triggering the respective build time safety
    check.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/flushtlb.c    | 9 +++++----
 xen/include/asm-x86/nops.h | 2 ++
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/flushtlb.c b/xen/arch/x86/flushtlb.c
index 90a004f..961bb1d 100644
--- a/xen/arch/x86/flushtlb.c
+++ b/xen/arch/x86/flushtlb.c
@@ -141,10 +141,11 @@ 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 " __stringify(NOP_DS_PREFIX) ";"
+                                  " clflush %0",
+                                  "data16 clflush %0",      /* clflushopt */
+                                  X86_FEATURE_CLFLUSHOPT,
+                                  "m" (((const char *)va)[i]));
         }
         else
         {
diff --git a/xen/include/asm-x86/nops.h b/xen/include/asm-x86/nops.h
index 05f9162..1368a97 100644
--- a/xen/include/asm-x86/nops.h
+++ b/xen/include/asm-x86/nops.h
@@ -7,6 +7,8 @@
  * Define nops for use with alternative().
  */
 
+#define NOP_DS_PREFIX 0x3e
+
 /*
  * Opteron 64bit nops
  * 1: nop
--
generated by git-patchbot for /home/xen/git/xen.git#master

_______________________________________________
Xen-changelog mailing list
Xen-changelog@xxxxxxxxxxxxx
http://lists.xensource.com/xen-changelog