x86/hvm: Use guest_cpuid() rather than hvm_cpuid()

More work is required before maxphysaddr can be read straight out of the
cpuid_policy block, but in the meantime hvm_cpuid() wants to disappear so
update the code to use the newer interface.

Use the behaviour of max_leaf handling (returning all zeros) to avoid a double
call into guest_cpuid().

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

xen/arch/x86/hvm/mtrr.c

@@ -440,7 +440,7 @@ bool_t mtrr_fix_range_msr_set(struct domain *d, struct mtrr_state *m,
 bool_t mtrr_var_range_msr_set(
     struct domain *d, struct mtrr_state *m, uint32_t msr, uint64_t msr_content)
 {
-    uint32_t index, phys_addr, eax;
+    uint32_t index, phys_addr;
     uint64_t msr_mask;
     uint64_t *var_range_base = (uint64_t*)m->var_ranges;
 
@@ -453,13 +453,10 @@ bool_t mtrr_var_range_msr_set(
 
     if ( d == current->domain )
     {
-        phys_addr = 36;
-        hvm_cpuid(0x80000000, &eax, NULL, NULL, NULL);
-        if ( (eax >> 16) == 0x8000 && eax >= 0x80000008 )
-        {
-            hvm_cpuid(0x80000008, &eax, NULL, NULL, NULL);
-            phys_addr = (uint8_t)eax;
-        }
+        struct cpuid_leaf res;
+
+        guest_cpuid(current, 0x80000008, 0, &res);
+        phys_addr = (uint8_t)res.a ?: 36;
     }
     else
         phys_addr = paddr_bits;