target/i386: Add VMX_SECONDARY_EXEC_MODE_BASED_EPT_EXEC

Enumerate ability to enable Intel Mode-Based Execute Control (MBEC)
on secondary execution control bit 22.

Intel MBEC is a hardware feature, introduced in the Kabylake
generation, that allows for more granular control over execution
permissions. MBEC enables the separation and tracking of execution
permissions for supervisor (kernel) and user-mode code. It is used as
an accelerator for Microsoft's Memory Integrity [1] (also known as
hypervisor-protected code integrity or HVCI).

[1] https://learn.microsoft.com/en-us/windows/security/hardware-security/enable-virtualization-based-protection-of-code-integrity

Code is mirrored here:
https://github.com/JonKohler/linux/tree/mbec-v1-6.18
https://github.com/JonKohler/kvm-unit-tests/tree/mbec-v1

LKML thread(s) are here:
Original RFC: https://lore.kernel.org/all/20250313203702.575156-1-jon@nutanix.com/
V1 code: https://lore.kernel.org/all/20251223054806.1611168-1-jon@nutanix.com/
KVM unit test changes: https://lore.kernel.org/all/20251223054850.1611618-1-jon@nutanix.com/

Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Zhao Liu <zhao1.liu@intel.com>
Co-authored-by: Jon Kohler <jon@nutanix.com>
Co-authored-by: Aditya Desai <aditya.desai@nutanix.com>
Signed-off-by: Jon Kohler <jon@nutanix.com>
Link: https://lore.kernel.org/r/20251223060834.1618428-1-jon@nutanix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/cpu.c

@@ -1656,7 +1656,7 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
             "vmx-apicv-register", "vmx-apicv-vid", "vmx-ple", "vmx-rdrand-exit",
             "vmx-invpcid-exit", "vmx-vmfunc", "vmx-shadow-vmcs", "vmx-encls-exit",
             "vmx-rdseed-exit", "vmx-pml", NULL, NULL,
-            "vmx-xsaves", NULL, NULL, NULL,
+            "vmx-xsaves", NULL, "vmx-mbec", NULL,
             NULL, "vmx-tsc-scaling", "vmx-enable-user-wait-pause", NULL,
             NULL, NULL, NULL, NULL,
         },
@@ -1971,6 +1971,10 @@ static FeatureDep feature_dependencies[] = {
         .from = { FEAT_VMX_SECONDARY_CTLS,  VMX_SECONDARY_EXEC_ENABLE_EPT },
         .to = { FEAT_VMX_SECONDARY_CTLS,    VMX_SECONDARY_EXEC_UNRESTRICTED_GUEST },
     },
+    {
+        .from = { FEAT_VMX_SECONDARY_CTLS,  VMX_SECONDARY_EXEC_ENABLE_EPT },
+        .to = { FEAT_VMX_SECONDARY_CTLS,    VMX_SECONDARY_EXEC_MODE_BASED_EPT_EXEC },
+    },
     {
         .from = { FEAT_VMX_SECONDARY_CTLS,  VMX_SECONDARY_EXEC_ENABLE_VPID },
         .to = { FEAT_VMX_EPT_VPID_CAPS,     0xffffffffull << 32 },

target/i386/cpu.h

@@ -1414,6 +1414,7 @@ uint64_t x86_cpu_get_supported_feature_word(X86CPU *cpu, FeatureWord w);
 #define VMX_SECONDARY_EXEC_RDSEED_EXITING           0x00010000
 #define VMX_SECONDARY_EXEC_ENABLE_PML               0x00020000
 #define VMX_SECONDARY_EXEC_XSAVES                   0x00100000
+#define VMX_SECONDARY_EXEC_MODE_BASED_EPT_EXEC      0x00400000
 #define VMX_SECONDARY_EXEC_TSC_SCALING              0x02000000
 #define VMX_SECONDARY_EXEC_ENABLE_USER_WAIT_PAUSE   0x04000000