[Xen-devel] [PATCH 6/6] x86_emulate: fold wide reads

["Jan Beulich" <JBeulich@xxxxxxxx>]

With HVM's MMIO operand handling now being capable of splitting large
reads, there's no need to issue at most machine word size reads when
we really need wider operands.

Not that this is not done everywhere - there are a couple of cases
where keeping the reads separate is more natural (and folding them
would complicate the code rather than simplifying it).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1116,7 +1116,6 @@ protmode_load_seg(
 {
     struct segment_register desctab, ss, segr;
     struct { uint32_t a, b; } desc;
-    unsigned long val;
     uint8_t dpl, rpl, cpl;
     uint32_t new_desc_b, a_flag = 0x100;
     int rc, fault_type = EXC_GP;
@@ -1143,14 +1142,9 @@ protmode_load_seg(
     if ( ((sel & 0xfff8) + 7) > desctab.limit )
         goto raise_exn;
 
-    if ( (rc = read_ulong(x86_seg_none, desctab.base + (sel & 0xfff8),
-                          &val, 4, ctxt, ops)) )
+    if ( (rc = ops->read(x86_seg_none, desctab.base + (sel & 0xfff8),
+                         &desc, sizeof(desc), ctxt)) )
         return rc;
-    desc.a = val;
-    if ( (rc = read_ulong(x86_seg_none, desctab.base + (sel & 0xfff8) + 4,
-                          &val, 4, ctxt, ops)) )
-        return rc;
-    desc.b = val;
 
     /* Segment present in memory? */
     if ( !(desc.b & (1u<<15)) )
@@ -4454,7 +4448,6 @@ x86_emulate(
 
     case 0xc7: /* Grp9 (cmpxchg8b/cmpxchg16b) */ {
         unsigned long old[2], exp[2], new[2];
-        unsigned int i;
 
         generate_exception_if((modrm_reg & 7) != 1, EXC_UD, -1);
         generate_exception_if(ea.type != OP_MEM, EXC_UD, -1);
@@ -4463,10 +4456,9 @@ x86_emulate(
         op_bytes *= 2;
 
         /* Get actual old value. */
-        for ( i = 0; i < (op_bytes/sizeof(long)); i++ )
-            if ( (rc = read_ulong(ea.mem.seg, ea.mem.off + i*sizeof(long),
-                                  &old[i], sizeof(long), ctxt, ops)) != 0 )
-                goto done;
+        if ( (rc = ops->read(ea.mem.seg, ea.mem.off, old, op_bytes,
+                             ctxt)) != 0 )
+            goto done;
 
         /* Get expected and proposed values. */
         if ( op_bytes == 8 )

Attachment: x86-emul-fold-reads.patch
Description: Text document

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