x86: please Clang in arch_set_info_guest()

Clang 10 reports

domain.c:1328:10: error: variable 'cr3_mfn' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    if ( !compat )
         ^~~~~~~
domain.c:1334:34: note: uninitialized use occurs here
    cr3_page = get_page_from_mfn(cr3_mfn, d);
                                 ^~~~~~~
domain.c:1328:5: note: remove the 'if' if its condition is always true
    if ( !compat )
    ^~~~~~~~~~~~~~
domain.c:1042:18: note: initialize the variable 'cr3_mfn' to silence this warning
    mfn_t cr3_mfn;
                 ^
                  = 0
domain.c:1189:14: error: variable 'fail' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
        if ( !compat )
             ^~~~~~~
domain.c:1211:9: note: uninitialized use occurs here
        fail |= v->arch.pv.gdt_ents != c(gdt_ents);
        ^~~~
domain.c:1189:9: note: remove the 'if' if its condition is always true
        if ( !compat )
        ^~~~~~~~~~~~~~
domain.c:1187:18: note: initialize the variable 'fail' to silence this warning
        bool fail;
                 ^
                  = false

despite this being a build with -O2 in effect, and despite "compat"
being constant "false" when CONFIG_COMPAT (and hence CONFIG_PV32) is not
defined, as it gets set at the top of the function from the result of
is_pv_32bit_domain().

Re-arrange the two "offending" if()s such that when COMPAT=n the
respective variables will be seen as unconditionally initialized. The
original aim was to have the !compat cases first, though.

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

xen/arch/x86/domain.c

@@ -1186,7 +1186,17 @@ int arch_set_info_guest(
         unsigned long pfn = pagetable_get_pfn(v->arch.guest_table);
         bool fail;
 
-        if ( !compat )
+#ifdef CONFIG_COMPAT
+        if ( compat )
+        {
+            l4_pgentry_t *l4tab = map_domain_page(_mfn(pfn));
+
+            pfn = l4e_get_pfn(*l4tab);
+            unmap_domain_page(l4tab);
+            fail = compat_pfn_to_cr3(pfn) != c.cmp->ctrlreg[3];
+        }
+        else
+#endif
         {
             fail = xen_pfn_to_cr3(pfn) != c.nat->ctrlreg[3];
             if ( pagetable_is_null(v->arch.guest_table_user) )
@@ -1197,16 +1207,6 @@ int arch_set_info_guest(
                 fail |= xen_pfn_to_cr3(pfn) != c.nat->ctrlreg[1];
             }
         }
-#ifdef CONFIG_COMPAT
-        else
-        {
-            l4_pgentry_t *l4tab = map_domain_page(_mfn(pfn));
-
-            pfn = l4e_get_pfn(*l4tab);
-            unmap_domain_page(l4tab);
-            fail = compat_pfn_to_cr3(pfn) != c.cmp->ctrlreg[3];
-        }
-#endif
 
         fail |= v->arch.pv.gdt_ents != c(gdt_ents);
         for ( i = 0; !fail && i < nr_gdt_frames; ++i )
@@ -1325,12 +1325,12 @@ int arch_set_info_guest(
 
     set_bit(_VPF_in_reset, &v->pause_flags);
 
-    if ( !compat )
-        cr3_mfn = _mfn(xen_cr3_to_pfn(c.nat->ctrlreg[3]));
 #ifdef CONFIG_COMPAT
-    else
+    if ( compat )
         cr3_mfn = _mfn(compat_cr3_to_pfn(c.cmp->ctrlreg[3]));
+    else
 #endif
+        cr3_mfn = _mfn(xen_cr3_to_pfn(c.nat->ctrlreg[3]));
     cr3_page = get_page_from_mfn(cr3_mfn, d);
 
     if ( !cr3_page )