x86/spec-ctrl: Expose BHI_CTRL to guests

The CPUID feature bit signals the presence of the BHI_DIS_S control 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>
(cherry picked from commit 583f1d0950)

tools/misc/xen-cpuid.c

@@ -208,6 +208,7 @@ static const char *const str_7d2[32] =
 {
     [ 0] = "intel-psfd",    [ 1] = "ipred-ctrl",
     [ 2] = "rrsba-ctrl",
+    [ 4] = "bhi-ctrl",
 };
 
 static const char *const str_m10Al[32] =

xen/arch/x86/msr.c

@@ -325,6 +325,7 @@ uint64_t msr_spec_ctrl_valid_bits(const struct cpu_policy *cp)
              ? (SPEC_CTRL_IPRED_DIS_U | SPEC_CTRL_IPRED_DIS_S) : 0) |
             (cp->feat.rrsba_ctrl
              ? (SPEC_CTRL_RRSBA_DIS_U | SPEC_CTRL_RRSBA_DIS_S) : 0) |
+            (cp->feat.bhi_ctrl   ? SPEC_CTRL_BHI_DIS_S : 0) |
             0);
 }
 

xen/include/asm-x86/msr-index.h

@@ -41,6 +41,7 @@
 #define  SPEC_CTRL_RRSBA_DIS_U              (_AC(1, ULL) <<  5)
 #define  SPEC_CTRL_RRSBA_DIS_S              (_AC(1, ULL) <<  6)
 #define  SPEC_CTRL_PSFD                     (_AC(1, ULL) <<  7)
+#define  SPEC_CTRL_BHI_DIS_S                (_AC(1, ULL) << 10)
 
 #define MSR_PRED_CMD                        0x00000049
 #define  PRED_CMD_IBPB                      (_AC(1, ULL) <<  0)

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

@@ -303,6 +303,7 @@ XEN_CPUFEATURE(SRSO_NO,            11*32+29) /*A  Hardware not vulenrable to Spe
 XEN_CPUFEATURE(INTEL_PSFD,         13*32+ 0) /*A  MSR_SPEC_CTRL.PSFD */
 XEN_CPUFEATURE(IPRED_CTRL,         13*32+ 1) /*A  MSR_SPEC_CTRL.IPRED_DIS_* */
 XEN_CPUFEATURE(RRSBA_CTRL,         13*32+ 2) /*A  MSR_SPEC_CTRL.RRSBA_DIS_* */
+XEN_CPUFEATURE(BHI_CTRL,           13*32+ 4) /*A  MSR_SPEC_CTRL.BHI_DIS_S */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1.ecx, word 14 */
 

xen/tools/gen-cpuid.py

@@ -316,7 +316,7 @@ def crunch_numbers(state):
         # as dependent features simplifies Xen's logic, and prevents the guest
         # from seeing implausible configurations.
         IBRSB: [STIBP, SSBD, INTEL_PSFD, EIBRS,
-                IPRED_CTRL, RRSBA_CTRL],
+                IPRED_CTRL, RRSBA_CTRL, BHI_CTRL],
         IBRS: [AMD_STIBP, AMD_SSBD, PSFD,
                IBRS_ALWAYS, IBRS_FAST, IBRS_SAME_MODE],
         IBPB: [IBPB_RET, SBPB, IBPB_BRTYPE],