x86/spec-ctrl: Drop use_spec_ctrl boolean

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@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit ec083bf552c35e10347449e21809f4780f8155d2)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index f70535b6e7bfee21963b1a575440749d116846d4..e85b0c0c7dffd8a31522247b93bffdc29e7ec180 100644 (file)
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -898,7 +898,7 @@ 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;
 
@@ -987,19 +987,21 @@ 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;
     }