Subject: x86/cpu-policy: Simplify logic in guest_common_default_feature_adjustments()
From: Andrew Cooper <[email-masked]>
Date: Fri, 27 Jun 2025 14:46:01 +0100

For features which are unconditionally set in the max policies, making the
default policy to match the host can be done with a conditional clear.

This is simpler than the unconditional clear, conditional set currently
performed.

Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>
(cherry picked from commit 30f8fed68f3c2e63594ff9202b3d05b971781e36)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index c813df35cbb0..42bd039e7c2e 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -511,17 +511,14 @@ static void __init guest_common_default_feature_adjustments(uint32_t *fs)
          * reasons, so reset the default policy back to the host values in
          * case we're unaffected.
          */
-        __clear_bit(X86_FEATURE_MD_CLEAR, fs);
-        if ( cpu_has_md_clear )
-            __set_bit(X86_FEATURE_MD_CLEAR, fs);
+        if ( !cpu_has_md_clear )
+            __clear_bit(X86_FEATURE_MD_CLEAR, fs);
 
-        __clear_bit(X86_FEATURE_FB_CLEAR, fs);
-        if ( cpu_has_fb_clear )
-            __set_bit(X86_FEATURE_FB_CLEAR, fs);
+        if ( !cpu_has_fb_clear )
+            __clear_bit(X86_FEATURE_FB_CLEAR, fs);
 
-        __clear_bit(X86_FEATURE_RFDS_CLEAR, fs);
-        if ( cpu_has_rfds_clear )
-            __set_bit(X86_FEATURE_RFDS_CLEAR, fs);
+        if ( !cpu_has_rfds_clear )
+            __clear_bit(X86_FEATURE_RFDS_CLEAR, fs);
 
         /*
          * The Gather Data Sampling microcode mitigation (August 2023) has an
@@ -541,13 +538,11 @@ static void __init guest_common_default_feature_adjustments(uint32_t *fs)
      * Topology information is at the toolstack's discretion so these are
      * unconditionally set in max, but pick a default which matches the host.
      */
-    __clear_bit(X86_FEATURE_HTT, fs);
-    if ( cpu_has_htt )
-        __set_bit(X86_FEATURE_HTT, fs);
+    if ( !cpu_has_htt )
+        __clear_bit(X86_FEATURE_HTT, fs);
 
-    __clear_bit(X86_FEATURE_CMP_LEGACY, fs);
-    if ( cpu_has_cmp_legacy )
-        __set_bit(X86_FEATURE_CMP_LEGACY, fs);
+    if ( !cpu_has_cmp_legacy )
+        __clear_bit(X86_FEATURE_CMP_LEGACY, fs);
 
     /*
      * On certain hardware, speculative or errata workarounds can result in