x86/spec-ctrl: Introduce new has_spec_ctrl boolean

Most MSR_SPEC_CTRL setup will be common between Intel and AMD.  Instead of
opencoding an OR of two features everywhere, introduce has_spec_ctrl instead.

Reword the comment above the Intel specific alternatives block to highlight
that it is Intel specific, and pull the setting of default_xen_spec_ctrl.IBRS
out because it will want to be common.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 5d9eff3a312763d889cfbf3c8468b6dfb3ab490c)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index a072ec987793a91fb80832fdc46ed65362c3098c..422714f3a2114bd642eb87b3b54112abfed3dfda 100644 (file)
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -923,7 +923,7 @@ void spec_ctrl_init_domain(struct domain *d)
 void __init init_speculation_mitigations(void)
 {
     enum ind_thunk thunk = THUNK_DEFAULT;
-    bool ibrs = false, hw_smt_enabled;
+    bool has_spec_ctrl, ibrs = false, hw_smt_enabled;
     bool cpu_has_bug_taa;
     uint64_t caps = 0;
 
@@ -932,6 +932,8 @@ void __init init_speculation_mitigations(void)
 
     hw_smt_enabled = check_smt_enabled();
 
+    has_spec_ctrl = boot_cpu_has(X86_FEATURE_IBRSB);
+
     /*
      * Has the user specified any custom BTI mitigations?  If so, follow their
      * instructions exactly and disable all heuristics.
@@ -955,11 +957,11 @@ void __init init_speculation_mitigations(void)
              */
             if ( retpoline_safe(caps) )
                 thunk = THUNK_RETPOLINE;
-            else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+            else if ( has_spec_ctrl )
                 ibrs = true;
         }
         /* Without compiler thunk support, use IBRS if available. */
-        else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+        else if ( has_spec_ctrl )
             ibrs = true;
     }
 
@@ -990,10 +992,7 @@ void __init init_speculation_mitigations(void)
     else if ( thunk == THUNK_JMP )
         setup_force_cpu_cap(X86_FEATURE_IND_THUNK_JMP);
 
-    /*
-     * If we are on hardware supporting MSR_SPEC_CTRL, see about setting up
-     * the alternatives blocks so we can virtualise support for guests.
-     */
+    /* Intel hardware: MSR_SPEC_CTRL alternatives setup. */
     if ( boot_cpu_has(X86_FEATURE_IBRSB) )
     {
         if ( opt_msr_sc_pv )
@@ -1012,11 +1011,12 @@ void __init init_speculation_mitigations(void)
             default_spec_ctrl_flags |= SCF_ist_wrmsr;
             setup_force_cpu_cap(X86_FEATURE_SC_MSR_HVM);
         }
-
-        if ( ibrs )
-            default_xen_spec_ctrl |= SPEC_CTRL_IBRS;
     }
 
+    /* If we have IBRS available, see whether we should use it. */
+    if ( has_spec_ctrl && ibrs )
+        default_xen_spec_ctrl |= SPEC_CTRL_IBRS;
+
     /* If we have SSBD available, see whether we should use it. */
     if ( boot_cpu_has(X86_FEATURE_SSBD) && opt_ssbd )
         default_xen_spec_ctrl |= SPEC_CTRL_SSBD;
@@ -1250,7 +1250,7 @@ void __init init_speculation_mitigations(void)
      * boot won't have any other code running in a position to mount an
      * attack.
      */
-    if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+    if ( has_spec_ctrl )
     {
         bsp_delay_spec_ctrl = !cpu_has_hypervisor && default_xen_spec_ctrl;