File 5b0d2ddc-x86-CPUID-dont-override-tool-stack-hidden-STIBP.patch of Package xen.8389

# Commit 06f542f8f2e446c01bd0edab51e9450af7f6e05b
# Date 2018-05-29 12:39:24 +0200
# Author Jan Beulich <jbeulich@suse.com>
# Committer Jan Beulich <jbeulich@suse.com>
x86/CPUID: don't override tool stack decision to hide STIBP

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 to not
expose it (for whatever [questionable?] reason). That feature set side
override is sufficient to achieve the intended guest side safety
property (in offering - by default - STIBP independent of actual
availability in hardware).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -620,14 +620,6 @@ 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 &&
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -243,7 +243,7 @@ XEN_CPUFEATURE(IBPB,          8*32+12) /
 XEN_CPUFEATURE(AVX512_4VNNIW, 9*32+ 2) /*A  AVX512 Neural Network Instructions */
 XEN_CPUFEATURE(AVX512_4FMAPS, 9*32+ 3) /*A  AVX512 Multiply Accumulation Single Precision */
 XEN_CPUFEATURE(IBRSB,         9*32+26) /*A  IBRS and IBPB support (used by Intel) */
-XEN_CPUFEATURE(STIBP,         9*32+27) /*A! STIBP */
+XEN_CPUFEATURE(STIBP,         9*32+27) /*A  STIBP */
 XEN_CPUFEATURE(ARCH_CAPS,     9*32+29) /*   IA32_ARCH_CAPABILITIES MSR */
 XEN_CPUFEATURE(SSBD,          9*32+31) /*A  MSR_SPEC_CTRL.SSBD available */