x86/spec-ctrl: Expose IPRED_CTRL to guests

The CPUID feature bit signals the presence of the IPRED_DIS_{U,S} controls in
SPEC_CTRL MSR, first available in Intel AlderLake and Sapphire Rapids CPUs.

Xen already knows how to context switch MSR_SPEC_CTRL properly between guest
and hypervisor context.

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

xen/arch/x86/msr.c

@@ -309,8 +309,8 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val)
 
 /*
  * Caller to confirm that MSR_SPEC_CTRL is available.  Intel and AMD have
- * separate CPUID features for this functionality, but only set will be
- * active.
+ * separate CPUID features for some of this functionality, but only one
+ * vendors-worth will be active on a single host.
  */
 uint64_t msr_spec_ctrl_valid_bits(const struct cpu_policy *cp)
 {
@@ -324,6 +324,8 @@ uint64_t msr_spec_ctrl_valid_bits(const struct cpu_policy *cp)
     return (SPEC_CTRL_IBRS | SPEC_CTRL_STIBP |
             (ssbd       ? SPEC_CTRL_SSBD       : 0) |
             (psfd       ? SPEC_CTRL_PSFD       : 0) |
+            (cp->feat.ipred_ctrl
+             ? (SPEC_CTRL_IPRED_DIS_U | SPEC_CTRL_IPRED_DIS_S) : 0) |
             0);
 }
 

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

@@ -302,7 +302,7 @@ XEN_CPUFEATURE(INTEL_PPIN,         12*32+ 0) /*   Protected Processor Inventory
 
 /* Intel-defined CPU features, CPUID level 0x00000007:2.edx, word 13 */
 XEN_CPUFEATURE(INTEL_PSFD,         13*32+ 0) /*A  MSR_SPEC_CTRL.PSFD */
-XEN_CPUFEATURE(IPRED_CTRL,         13*32+ 1) /*   MSR_SPEC_CTRL.IPRED_DIS_* */
+XEN_CPUFEATURE(IPRED_CTRL,         13*32+ 1) /*A  MSR_SPEC_CTRL.IPRED_DIS_* */
 XEN_CPUFEATURE(RRSBA_CTRL,         13*32+ 2) /*   MSR_SPEC_CTRL.RRSBA_DIS_* */
 XEN_CPUFEATURE(DDP_CTRL,           13*32+ 3) /*   MSR_SPEC_CTRL.DDP_DIS_U */
 XEN_CPUFEATURE(BHI_CTRL,           13*32+ 4) /*   MSR_SPEC_CTRL.BHI_DIS_S */

xen/tools/gen-cpuid.py

@@ -319,7 +319,8 @@ def crunch_numbers(state):
         # IBRSB/IBRS, and we pass this MSR directly to guests.  Treating them
         # as dependent features simplifies Xen's logic, and prevents the guest
         # from seeing implausible configurations.
-        IBRSB: [STIBP, SSBD, INTEL_PSFD, EIBRS],
+        IBRSB: [STIBP, SSBD, INTEL_PSFD, EIBRS,
+                IPRED_CTRL],
         IBRS: [AMD_STIBP, AMD_SSBD, PSFD, AUTO_IBRS,
                IBRS_ALWAYS, IBRS_FAST, IBRS_SAME_MODE],
         IBPB: [IBPB_RET, SBPB, IBPB_BRTYPE],