x86/spec: fix INDIRECT_THUNK option to only be set when build-enabled

Attempt to provide a more helpful error message when the user attempts to set
spec-ctrl=bti-thunk option but the support is build-time disabled.

While there also adjust the command line documentation to mention
CONFIG_INDIRECT_THUNK instead of INDIRECT_THUNK.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 8441fa806a
master date: 2024-02-27 14:58:20 +0100

docs/misc/xen-command-line.pandoc

@@ -2424,11 +2424,11 @@ guests to use.
   performance reasons dom0 is unprotected by default.  If it is necessary to
   protect dom0 too, boot with `spec-ctrl=ibpb-entry`.
 
-If Xen was compiled with INDIRECT_THUNK support, `bti-thunk=` can be used to
-select which of the thunks gets patched into the `__x86_indirect_thunk_%reg`
-locations.  The default thunk is `retpoline` (generally preferred), with the
-alternatives being `jmp` (a `jmp *%reg` gadget, minimal overhead), and
-`lfence` (an `lfence; jmp *%reg` gadget).
+If Xen was compiled with `CONFIG_INDIRECT_THUNK` support, `bti-thunk=` can be
+used to select which of the thunks gets patched into the
+`__x86_indirect_thunk_%reg` locations.  The default thunk is `retpoline`
+(generally preferred), with the alternatives being `jmp` (a `jmp *%reg` gadget,
+minimal overhead), and `lfence` (an `lfence; jmp *%reg` gadget).
 
 On hardware supporting IBRS (Indirect Branch Restricted Speculation), the
 `ibrs=` option can be used to force or prevent Xen using the feature itself.

xen/arch/x86/spec_ctrl.c

@@ -241,7 +241,12 @@ static int __init cf_check parse_spec_ctrl(const char *s)
         {
             s += 10;
 
-            if ( !cmdline_strcmp(s, "retpoline") )
+            if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
+            {
+                no_config_param("INDIRECT_THUNK", "spec-ctrl", s - 10, ss);
+                rc = -EINVAL;
+            }
+            else if ( !cmdline_strcmp(s, "retpoline") )
                 opt_thunk = THUNK_RETPOLINE;
             else if ( !cmdline_strcmp(s, "lfence") )
                 opt_thunk = THUNK_LFENCE;