x86/spec-ctrl: Add fine-grained cmdline suboptions for primitives

Support controling the PV/HVM suboption of msr-sc/rsb/md-clear, which
previously wasn't possible.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 27357c394b)

docs/misc/xen-command-line.pandoc

@@ -2104,7 +2104,8 @@ not be able to control the state of the mitigation.
 By default SSBD will be mitigated at runtime (i.e `ssbd=runtime`).
 
 ### spec-ctrl (x86)
-> `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb,md-clear}=<bool>,
+> `= List of [ <bool>, xen=<bool>, {pv,hvm}=<bool>,
+>              {msr-sc,rsb,md-clear}=<bool>|{pv,hvm}=<bool>,
 >              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,psfd,
 >              eager-fpu,l1d-flush,branch-harden,srb-lock,
 >              unpriv-mmio}=<bool> ]`
@@ -2129,12 +2130,17 @@ 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=`, `rsb=` and `md-clear=` offer fine
+The `pv=`, `hvm=`, `msr-sc=`, `rsb=` and `md-clear=` options offer fine
 grained control over the primitives by Xen.  These impact Xen's ability to
-protect itself, and Xen's ability to virtualise support for guests to use.
+protect itself, and/or Xen's ability to virtualise support for guests to use.
 
 * `pv=` and `hvm=` offer control over all suboptions for PV and HVM guests
   respectively.
+* Each other option can be used either as a plain boolean
+  (e.g. `spec-ctrl=rsb` to control both the PV and HVM sub-options), or with
+  `pv=` or `hvm=` subsuboptions (e.g. `spec-ctrl=rsb=no-hvm` to disable HVM
+  RSB only).
+
 * `msr-sc=` offers control over Xen's support for manipulating `MSR_SPEC_CTRL`
   on entry and exit.  These blocks are necessary to virtualise support for
   guests and if disabled, guests will be unable to use IBRS/STIBP/SSBD/etc.

xen/arch/x86/spec_ctrl.c

@@ -147,20 +147,68 @@ static int __init parse_spec_ctrl(const char *s)
             opt_rsb_hvm = val;
             opt_md_clear_hvm = val;
         }
-        else if ( (val = parse_boolean("msr-sc", s, ss)) >= 0 )
+        else if ( (val = parse_boolean("msr-sc", s, ss)) != -1 )
         {
-            opt_msr_sc_pv = val;
-            opt_msr_sc_hvm = val;
+            switch ( val )
+            {
+            case 0:
+            case 1:
+                opt_msr_sc_pv = opt_msr_sc_hvm = val;
+                break;
+
+            case -2:
+                s += strlen("msr-sc=");
+                if ( (val = parse_boolean("pv", s, ss)) >= 0 )
+                    opt_msr_sc_pv = val;
+                else if ( (val = parse_boolean("hvm", s, ss)) >= 0 )
+                    opt_msr_sc_hvm = val;
+                else
+            default:
+                    rc = -EINVAL;
+                break;
+            }
         }
-        else if ( (val = parse_boolean("rsb", s, ss)) >= 0 )
+        else if ( (val = parse_boolean("rsb", s, ss)) != -1 )
         {
-            opt_rsb_pv = val;
-            opt_rsb_hvm = val;
+            switch ( val )
+            {
+            case 0:
+            case 1:
+                opt_rsb_pv = opt_rsb_hvm = val;
+                break;
+
+            case -2:
+                s += strlen("rsb=");
+                if ( (val = parse_boolean("pv", s, ss)) >= 0 )
+                    opt_rsb_pv = val;
+                else if ( (val = parse_boolean("hvm", s, ss)) >= 0 )
+                    opt_rsb_hvm = val;
+                else
+            default:
+                    rc = -EINVAL;
+                break;
+            }
         }
-        else if ( (val = parse_boolean("md-clear", s, ss)) >= 0 )
+        else if ( (val = parse_boolean("md-clear", s, ss)) != -1 )
         {
-            opt_md_clear_pv = val;
-            opt_md_clear_hvm = val;
+            switch ( val )
+            {
+            case 0:
+            case 1:
+                opt_md_clear_pv = opt_md_clear_hvm = val;
+                break;
+
+            case -2:
+                s += strlen("md-clear=");
+                if ( (val = parse_boolean("pv", s, ss)) >= 0 )
+                    opt_md_clear_pv = val;
+                else if ( (val = parse_boolean("hvm", s, ss)) >= 0 )
+                    opt_md_clear_hvm = val;
+                else
+            default:
+                    rc = -EINVAL;
+                break;
+            }
         }
 
         /* Xen's speculative sidechannel mitigation settings. */