x86: Make "spec-ctrl=no" a global disable of all mitigations

In order to have a simple and easy to remember means to suppress all the
more or less recent workarounds for hardware vulnerabilities, force
settings not controlled by "spec-ctrl=" also to their original defaults,
unless they've been forced to specific values already by earlier command
line options.

This is part of XSA-273.

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

docs/misc/xen-command-line.markdown

@@ -1838,10 +1838,15 @@ extreme care.**
 
 An overall boolean value, `spec-ctrl=no`, can be specified to turn off all
 mitigations, including pieces of infrastructure used to virtualise certain
-mitigation features for guests.  Alternatively, a slightly more restricted
-`spec-ctrl=no-xen` can be used to turn off all of Xen's mitigations, while
-leaving the virtualisation support in place for guests to use.  Use of a
-positive boolean value for either of these options is invalid.
+mitigation features for guests.  This also includes settings which `xpti`,
+`smt`, `pv-l1tf` control, unless the respective option(s) have been
+specified earlier on the command line.
+
+Alternatively, a slightly more restricted `spec-ctrl=no-xen` can be used to
+turn off all of Xen's mitigations, while leaving the virtualisation support
+in place for guests to use.
+
+Use of a positive boolean value for either of these options is invalid.
 
 The booleans `pv=`, `hvm=`, `msr-sc=` and `rsb=` offer fine grained control
 over the alternative blocks used by Xen.  These impact Xen's ability to

xen/arch/x86/spec_ctrl.c

@@ -134,6 +134,15 @@ static int __init parse_spec_ctrl(const char *s)
 
             opt_eager_fpu = 0;
 
+            if ( opt_xpti < 0 )
+                opt_xpti = 0;
+
+            if ( opt_smt < 0 )
+                opt_smt = 1;
+
+            if ( opt_pv_l1tf < 0 )
+                opt_pv_l1tf = 0;
+
         disable_common:
             opt_rsb_pv = false;
             opt_rsb_hvm = false;