x86/vmx: Factor locate_msr_entry() out of vmx_find_msr() and vmx_add_msr()

Instead of having multiple algorithms searching the MSR lists, implement a
single one.  It has the semantics required by vmx_add_msr(), to identify the
position in which an MSR should live, if it isn't already present.

There will be a marginal improvement for vmx_find_msr() by avoiding the
function pointer calls to vmx_msr_entry_key_cmp(), and a major improvement for
vmx_add_msr() by using a binary search instead of a linear search.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>

xen/arch/x86/hvm/vmx/vmcs.c

@@ -1272,24 +1272,36 @@ static int construct_vmcs(struct vcpu *v)
     return rc;
 }
 
-static int vmx_msr_entry_key_cmp(const void *key, const void *elt)
+/*
+ * Search an MSR list looking for an MSR entry, or the slot in which it should
+ * live (to keep the data sorted) if an entry is not found.
+ *
+ * The return pointer is guaranteed to be bounded by start and end.  However,
+ * it may point at end, and may be invalid for the caller to dereference.
+ */
+static struct vmx_msr_entry *locate_msr_entry(
+    struct vmx_msr_entry *start, struct vmx_msr_entry *end, uint32_t msr)
 {
-    const u32 *msr = key;
-    const struct vmx_msr_entry *entry = elt;
+    while ( start < end )
+    {
+        struct vmx_msr_entry *mid = start + (end - start) / 2;
 
-    if ( *msr > entry->index )
-        return 1;
-    if ( *msr < entry->index )
-        return -1;
+        if ( msr < mid->index )
+            end = mid;
+        else if ( msr > mid->index )
+            start = mid + 1;
+        else
+            return mid;
+    }
 
-    return 0;
+    return start;
 }
 
 struct vmx_msr_entry *vmx_find_msr(uint32_t msr, enum vmx_msr_list_type type)
 {
     struct vcpu *curr = current;
     struct arch_vmx_struct *vmx = &curr->arch.hvm_vmx;
-    struct vmx_msr_entry *start = NULL;
+    struct vmx_msr_entry *start = NULL, *ent, *end;
     unsigned int total;
 
     switch ( type )
@@ -1311,7 +1323,10 @@ struct vmx_msr_entry *vmx_find_msr(uint32_t msr, enum vmx_msr_list_type type)
     if ( !start )
         return NULL;
 
-    return bsearch(&msr, start, total, sizeof(*start), vmx_msr_entry_key_cmp);
+    end = start + total;
+    ent = locate_msr_entry(start, end, msr);
+
+    return ((ent < end) && (ent->index == msr)) ? ent : NULL;
 }
 
 int vmx_add_msr(uint32_t msr, enum vmx_msr_list_type type)
@@ -1363,10 +1378,10 @@ int vmx_add_msr(uint32_t msr, enum vmx_msr_list_type type)
 
     start = *ptr;
     end   = start + total;
+    ent   = locate_msr_entry(start, end, msr);
 
-    for ( ent = start; ent < end && ent->index <= msr; ++ent )
-        if ( ent->index == msr )
-            return 0;
+    if ( (ent < end) && (ent->index == msr) )
+        return 0;
 
     if ( total == (PAGE_SIZE / sizeof(*ent)) )
         return -ENOSPC;