[Xen-changelog] [xen master] x86/CPUID: don't override tool stack decision to hide STIBP

[patchbot@xxxxxxx]

commit 06f542f8f2e446c01bd0edab51e9450af7f6e05b
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue May 29 12:39:24 2018 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue May 29 12:39:24 2018 +0200

    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@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Release-acked-by: Juergen Gross <jgross@xxxxxxxx>
---
 xen/arch/x86/cpuid.c                        | 8 --------
 xen/include/public/arch-x86/cpufeatureset.h | 2 +-
 2 files changed, 1 insertion(+), 9 deletions(-)

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index 4b8d33013f..beee47d0ed 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -642,14 +642,6 @@ void recalculate_cpuid_policy(struct domain *d)
     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 &&
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index c721c125ab..f1a5ed93e0 100644
--- 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) /*A  IBPB support 
only (no IBRS, used by
 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 */
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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