x86: use tzcnt instead of bsf

Following a compiler change done in 2012, make use of the fact that for
non-zero input BSF and TZCNT produce the same numeric result (EFLAGS
setting differs), and that CPUs not knowing of TZCNT will treat the
instruction as BSF (i.e. ignore what looks like a REP prefix to them).
The assumption here is that TZCNT would never have worse performance
than BSF.

Also extend the asm() input in find_first_set_bit() to allow memory
operands.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/xen/arch/x86/bitops.c b/xen/arch/x86/bitops.c
index c037567c6ed174b3791be844eb8314f7b4b4e952..f6ee71512cf841b223b80a20919edb5f66ff9c3c 100644 (file)
--- a/xen/arch/x86/bitops.c
+++ b/xen/arch/x86/bitops.c
@@ -62,7 +62,7 @@ unsigned int __find_first_zero_bit(
         "   je 2f\n\t"
         "   xor -"STR(BITS_PER_LONG/8)"(%2),%3\n\t"
         "   jz 1b\n\t"
-        "   bsf %3,%0\n\t"
+        "   rep; bsf %3,%0\n\t"
         "   lea -"STR(BITS_PER_LONG/8)"(%2),%2\n\t"
         "2: sub %%ebx,%%edi\n\t"
         "   shl $3,%%edi\n\t"

diff --git a/xen/arch/x86/hvm/vpic.c b/xen/arch/x86/hvm/vpic.c
index 2c6e6e5f3fb42e93fffae672627755d76f53462e..d72b6ff3fa625685f5cb22aabaee3968407b7d73 100644 (file)
--- a/xen/arch/x86/hvm/vpic.c
+++ b/xen/arch/x86/hvm/vpic.c
@@ -56,7 +56,7 @@ static int vpic_get_priority(struct hvm_hw_vpic *vpic, uint8_t mask)
         return VPIC_PRIO_NONE;
 
     /* prio = ffs(mask ROR vpic->priority_add); */
-    asm ( "ror %%cl,%b1 ; bsf %1,%0"
+    asm ( "ror %%cl,%b1 ; rep; bsf %1,%0"
           : "=r" (prio) : "q" ((uint32_t)mask), "c" (vpic->priority_add) );
     return prio;
 }

diff --git a/xen/include/asm-x86/bitops.h b/xen/include/asm-x86/bitops.h
index 099a74c3737f940d6166493d355c642fcf03658e..6f0f34bc57fe8108d21304597651b852ee5d4b1e 100644 (file)
--- a/xen/include/asm-x86/bitops.h
+++ b/xen/include/asm-x86/bitops.h
@@ -391,7 +391,7 @@ static inline unsigned int __scanbit(unsigned long val, unsigned long max)
  */
 static inline unsigned int find_first_set_bit(unsigned long word)
 {
-    asm ( "bsf %1,%0" : "=r" (word) : "r" (word) );
+    asm ( "rep; bsf %1,%0" : "=r" (word) : "rm" (word) );
     return (unsigned int)word;
 }