[PATCH 4/8] x86/spec-ctrl: Drop use_spec_ctrl boolean

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Several bugfixes have reduced the utility of this variable from it's original
purpose, and now all it does is aid in the setup of SCF_ist_wrmsr.

Simplify the logic by drop the variable, and doubling up the setting of
SCF_ist_wrmsr for the PV and HVM blocks, which will make the AMD SPEC_CTRL
support easier to follow.  Leave a comment explaining why SCF_ist_wrmsr is
still necessary for the VMExit case.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
---
 xen/arch/x86/spec_ctrl.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index c18cc8aa493a..8a550d0a0902 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -927,7 +927,7 @@ static __init void mds_calculations(uint64_t caps)
 void __init init_speculation_mitigations(void)
 {
     enum ind_thunk thunk = THUNK_DEFAULT;
-    bool use_spec_ctrl = false, ibrs = false, hw_smt_enabled;
+    bool ibrs = false, hw_smt_enabled;
     bool cpu_has_bug_taa;
     uint64_t caps = 0;
 
@@ -1016,19 +1016,21 @@ void __init init_speculation_mitigations(void)
     {
         if ( opt_msr_sc_pv )
         {
-            use_spec_ctrl = true;
+            default_spec_ctrl_flags |= SCF_ist_wrmsr;
             setup_force_cpu_cap(X86_FEATURE_SC_MSR_PV);
         }
 
         if ( opt_msr_sc_hvm )
         {
-            use_spec_ctrl = true;
+            /*
+             * While the guest MSR_SPEC_CTRL value is loaded/saved atomically,
+             * Xen's value is not restored atomically.  An early NMI hitting
+             * the VMExit path needs to restore Xen's value for safety.
+             */
+            default_spec_ctrl_flags |= SCF_ist_wrmsr;
             setup_force_cpu_cap(X86_FEATURE_SC_MSR_HVM);
         }
 
-        if ( use_spec_ctrl )
-            default_spec_ctrl_flags |= SCF_ist_wrmsr;
-
         if ( ibrs )
             default_xen_spec_ctrl |= SPEC_CTRL_IBRS;
     }
-- 
2.11.0