[PATCH 2/4] x86/hvm: Use for_each_set_bit() in hvm_emulate_writeback()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

... which is more consise than the opencoded form.

Also, for production VMs, ~100% of emulations are simple MOVs, so it is likely
that there are no segments to write back.

Furthermore, now that find_{first,next}_bit() are no longer in use, the
seg_reg_{accessed,dirty} fields aren't forced to be unsigned long, although
they do need to remain unsigned int because of __set_bit() elsewhere.

No practical change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Pulling current out into curr is good for code generation.  When using current
in the loop, GCC can't retain the calculation across the call to
hvm_set_segment_register() and is forced to re-read from the cpu_info block.

However, if curr is initialised, it's calculated even in the likely path...
---
 xen/arch/x86/hvm/emulate.c             | 20 ++++++++++----------
 xen/arch/x86/include/asm/hvm/emulate.h |  4 ++--
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index feb4792cc567..732bdbab25b0 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -2908,18 +2908,18 @@ void hvm_emulate_init_per_insn(
 void hvm_emulate_writeback(
     struct hvm_emulate_ctxt *hvmemul_ctxt)
 {
-    enum x86_segment seg;
+    struct vcpu *curr;
+    unsigned int dirty = hvmemul_ctxt->seg_reg_dirty;
 
-    seg = find_first_bit(&hvmemul_ctxt->seg_reg_dirty,
-                         ARRAY_SIZE(hvmemul_ctxt->seg_reg));
+    if ( likely(!dirty) )
+        return;
 
-    while ( seg < ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
-    {
-        hvm_set_segment_register(current, seg, &hvmemul_ctxt->seg_reg[seg]);
-        seg = find_next_bit(&hvmemul_ctxt->seg_reg_dirty,
-                            ARRAY_SIZE(hvmemul_ctxt->seg_reg),
-                            seg+1);
-    }
+    curr = current;
+
+    for_each_set_bit ( seg, dirty )
+        hvm_set_segment_register(curr, seg, &hvmemul_ctxt->seg_reg[seg]);
+
+    hvmemul_ctxt->seg_reg_dirty = 0;
 }
 
 /*
diff --git a/xen/arch/x86/include/asm/hvm/emulate.h 
b/xen/arch/x86/include/asm/hvm/emulate.h
index 29d679442e10..972cdf1fa0cf 100644
--- a/xen/arch/x86/include/asm/hvm/emulate.h
+++ b/xen/arch/x86/include/asm/hvm/emulate.h
@@ -36,8 +36,8 @@ struct hvm_emulate_ctxt {
     unsigned int insn_buf_bytes;
 
     struct segment_register seg_reg[10];
-    unsigned long seg_reg_accessed;
-    unsigned long seg_reg_dirty;
+    unsigned int seg_reg_accessed;
+    unsigned int seg_reg_dirty;
 
     /*
      * MFNs behind temporary mappings in the write callback.  The length is
-- 
2.39.2