x86/msr: Virtualise MSR_SPEC_CTRL.SSBD for guests to use

Almost all infrastructure is already in place.  Update the reserved bits
calculation in guest_wrmsr(), and offer SSBD to guests by default.

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

xen/arch/x86/msr.c

@@ -197,6 +197,8 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
 
     switch ( msr )
     {
+        uint64_t rsvd;
+
     case MSR_INTEL_PLATFORM_INFO:
     case MSR_ARCH_CAPABILITIES:
         /* Read-only */
@@ -232,8 +234,10 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
          * Note: SPEC_CTRL_STIBP is specified as safe to use (i.e. ignored)
          * when STIBP isn't enumerated in hardware.
          */
+        rsvd = ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP |
+                 (cp->feat.ssbd ? SPEC_CTRL_SSBD : 0));
 
-        if ( val & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP) )
+        if ( val & rsvd )
             goto gp_fault; /* Rsvd bit set? */
 
         vp->spec_ctrl.raw = val;
@@ -252,12 +256,12 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
 
     case MSR_INTEL_MISC_FEATURES_ENABLES:
     {
-        uint64_t rsvd = ~0ull;
         bool old_cpuid_faulting = vp->misc_features_enables.cpuid_faulting;
 
         if ( !vp->misc_features_enables.available )
             goto gp_fault;
 
+        rsvd = ~0ull;
         if ( dp->plaform_info.cpuid_faulting )
             rsvd &= ~MSR_MISC_FEATURES_CPUID_FAULTING;
 

xen/include/public/arch-x86/cpufeatureset.h

@@ -245,7 +245,7 @@ XEN_CPUFEATURE(AVX512_4FMAPS, 9*32+ 3) /*A  AVX512 Multiply Accumulation Single
 XEN_CPUFEATURE(IBRSB,         9*32+26) /*A  IBRS and IBPB support (used by Intel) */
 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) /*   MSR_SPEC_CTRL.SSBD available */
+XEN_CPUFEATURE(SSBD,          9*32+31) /*A  MSR_SPEC_CTRL.SSBD available */
 
 #endif /* XEN_CPUFEATURE */