x86/spec: adjust logic that elides lfence

It's currently too restrictive by just checking whether there's a BHB clearing
sequence selected.  It should instead check whether BHB clearing is used on
entry from PV or HVM specifically.

Switch to use opt_bhb_entry_{pv,hvm} instead, and then remove cpu_has_bhb_seq
since it no longer has any users.

Reported-by: Jan Beulich <jbeulich@suse.com>
Fixes: 954c983abc ('x86/spec-ctrl: Software BHB-clearing sequences')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 656ae8f109
master date: 2024-04-25 16:37:01 +0200

xen/arch/x86/include/asm/cpufeature.h

@@ -181,9 +181,6 @@
 #define cpu_bug_fpu_ptrs        boot_cpu_has(X86_BUG_FPU_PTRS)
 #define cpu_bug_null_seg        boot_cpu_has(X86_BUG_NULL_SEG)
 
-#define cpu_has_bhb_seq        (boot_cpu_has(X86_SPEC_BHB_TSX) ||       \
-                                boot_cpu_has(X86_SPEC_BHB_LOOPS))
-
 enum _cache_type {
     CACHE_TYPE_NULL = 0,
     CACHE_TYPE_DATA = 1,

xen/arch/x86/spec_ctrl.c

@@ -2340,7 +2340,7 @@ void __init init_speculation_mitigations(void)
          * unconditional WRMSR.  If we do have it, or we're not using any
          * prior conditional block, then it's safe to drop the LFENCE.
          */
-        if ( !cpu_has_bhb_seq &&
+        if ( !opt_bhb_entry_pv &&
              (boot_cpu_has(X86_FEATURE_SC_MSR_PV) ||
               !boot_cpu_has(X86_FEATURE_IBPB_ENTRY_PV)) )
             setup_force_cpu_cap(X86_SPEC_NO_LFENCE_ENTRY_PV);
@@ -2356,7 +2356,7 @@ void __init init_speculation_mitigations(void)
          * active in the block that is skipped when interrupting guest
          * context, then it's safe to drop the LFENCE.
          */
-        if ( !cpu_has_bhb_seq &&
+        if ( !opt_bhb_entry_pv &&
              (boot_cpu_has(X86_FEATURE_SC_MSR_PV) ||
               (!boot_cpu_has(X86_FEATURE_IBPB_ENTRY_PV) &&
                !boot_cpu_has(X86_FEATURE_SC_RSB_PV))) )
@@ -2368,7 +2368,7 @@ void __init init_speculation_mitigations(void)
          * A BHB sequence, if used, is the only conditional action, so if we
          * don't have it, we don't need the safety LFENCE.
          */
-        if ( !cpu_has_bhb_seq )
+        if ( !opt_bhb_entry_hvm )
             setup_force_cpu_cap(X86_SPEC_NO_LFENCE_ENTRY_VMX);
     }