x86/dom0: Improve dom0= useability

Having a pvh boolean isn't ideal.  If we gain a 3rd virtulsation mode,
what does `dom0=no-pvh` mean?

Change the syntax to be "dom0 = pv | pvh" which offers an option to more
obviously select PV mode.  Hide both options behind the relevent
CONFIG_* settings, and default to PVH mode when CONFIG_PV is compiled
out.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

docs/misc/xen-command-line.pandoc

@@ -637,21 +637,23 @@ trace feature is only enabled in debugging builds of Xen.
 Specify the bit width of the DMA heap.
 
 ### dom0
-    = List of [ pvh=<bool>, shadow=<bool>, verbose=<bool> ]
+    = List of [ pv | pvh, shadow=<bool>, verbose=<bool> ]
 
     Applicability: x86
 
 Controls for how dom0 is constructed on x86 systems.
 
-*   The `pvh` boolean controls whether dom0 is constructed as a PV or a PVH
-    guest.  The default is PV.  In addition, the following requirements must
-    be met:
+*   The `pv` and `pvh` options select the virtualisation mode of dom0.
+
+    The `pv` option is only available when `CONFIG_PV` is compiled in.  The
+    `pvh` option is only available when `CONFIG_HVM` is compiled in.  When
+    both options are compiled in, the default is PV.
+
+    In addition, the following requirements must be met:
 
     *   The dom0 kernel selected by the boot loader must be capable of the
         selected mode.
-    *   For a PV dom0, Xen must have been compiled with `CONFIG_PV` enabled.
-    *   For a PVH dom0, Xen must have been compiled with `CONFIG_HVM` enabled,
-        and the hardware must have VT-x/SVM extensions available.
+    *   For a PVH dom0, the hardware must have VT-x/SVM extensions available.
 
 *   The `shadow` boolean is only applicable when dom0 is constructed as a PVH
     guest, and controls whether dom0 uses hardware assisted paging, or shadow

xen/arch/x86/dom0_build.c

@@ -280,7 +280,7 @@ struct vcpu *__init alloc_dom0_vcpu0(struct domain *dom0)
 #ifdef CONFIG_SHADOW_PAGING
 bool __initdata opt_dom0_shadow;
 #endif
-bool __initdata dom0_pvh;
+bool __initdata dom0_pvh = !IS_ENABLED(CONFIG_PV);
 bool __initdata dom0_verbose;
 
 static int __init parse_dom0_param(const char *s)
@@ -295,8 +295,10 @@ static int __init parse_dom0_param(const char *s)
         if ( !ss )
             ss = strchr(s, '\0');
 
-        if ( (val = parse_boolean("pvh", s, ss)) >= 0 )
-            dom0_pvh = val;
+        if ( IS_ENABLED(CONFIG_PV) && !cmdline_strcmp(s, "pv") )
+            dom0_pvh = false;
+        else if ( IS_ENABLED(CONFIG_HVM) && !cmdline_strcmp(s, "pvh") )
+            dom0_pvh = true;
 #ifdef CONFIG_SHADOW_PAGING
         else if ( (val = parse_boolean("shadow", s, ss)) >= 0 )
             opt_dom0_shadow = val;