[Xen-devel] [PATCH] x86/CPUID: don't override tool stack decision to hide STIBP

["Jan Beulich" <JBeulich@xxxxxxxx>]

Other than in the feature sets, where we indeed want to offer the
feature even if not enumerated on hardware, we shouldn't dictate the
feature being available if tool stack or host admin have decided not
to expose it (for whatever [questionable?] reason).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
This is effectively accompanying the discussion rooted at the 4.8/4.7
patch at
https://lists.xenproject.org/archives/html/xen-devel/2018-05/msg01028.html 
dealing with a feature leveling issue.

--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -642,14 +642,6 @@ void recalculate_cpuid_policy(struct dom
     recalculate_xstate(p);
     recalculate_misc(p);
 
-    /*
-     * Override STIBP to match IBRS.  Guests can safely use STIBP
-     * functionality on non-HT hardware, but can't necesserily protect
-     * themselves from SP2/Spectre/Branch Target Injection if STIBP is hidden
-     * on HT-capable hardware.
-     */
-    p->feat.stibp = p->feat.ibrsb;
-
     for ( i = 0; i < ARRAY_SIZE(p->cache.raw); ++i )
     {
         if ( p->cache.subleaf[i].type >= 1 &&



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel