[Xen-devel] [PATCH v2 15/17] x86emul: adjust_bnd() should check XCR0

["Jan Beulich" <JBeulich@xxxxxxxx>]

Experimentally MPX instructions have been confirmed to behave as NOPs
unless both related XCR0 bits are set to 1. By implication branches
then also don't clear BNDn.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -2141,12 +2141,16 @@ static bool umip_active(struct x86_emula
 static void adjust_bnd(struct x86_emulate_ctxt *ctxt,
                        const struct x86_emulate_ops *ops, enum vex_pfx pfx)
 {
-    uint64_t bndcfg;
+    uint64_t xcr0, bndcfg;
     int rc;
 
     if ( pfx == vex_f2 || !cpu_has_mpx || !vcpu_has_mpx() )
         return;
 
+    if ( !ops->read_xcr || ops->read_xcr(0, &xcr0, ctxt) != X86EMUL_OKAY ||
+         !(xcr0 & XSTATE_BNDREGS) || !(xcr0 & XSTATE_BNDCSR) )
+        return;
+
     if ( !mode_ring0() )
         bndcfg = read_bndcfgu();
     else if ( !ops->read_msr ||




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