TDX advertises core crystal clock with cpuid[0x15] as 25MHz for TD
guests and it's unchangeable from VMM. As a result, TDX guest reads
the APIC timer as the same frequency, 25MHz.

While KVM's default emulated frequency for APIC bus is 1GHz, set the
APIC bus rate to match with TDX explicitly to ensure KVM provide correct
emulated APIC timer for TD guest.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
Changes in v6:
 - new patch;
---
 target/i386/kvm/tdx.c | 13 +++++++++++++
 target/i386/kvm/tdx.h |  3 +++
 2 files changed, 16 insertions(+)

diff --git a/target/i386/kvm/tdx.c b/target/i386/kvm/tdx.c
index 887a5324b439..94b9be62c5dd 100644
--- a/target/i386/kvm/tdx.c
+++ b/target/i386/kvm/tdx.c
@@ -234,6 +234,19 @@ int tdx_pre_create_vcpu(CPUState *cpu, Error **errp)
     init_vm = g_malloc0(sizeof(struct kvm_tdx_init_vm) +
                         sizeof(struct kvm_cpuid_entry2) * KVM_MAX_CPUID_ENTRIES);
 
+    if (!kvm_check_extension(kvm_state, KVM_CAP_X86_APIC_BUS_CYCLES_NS)) {
+        error_setg(errp, "KVM doesn't support KVM_CAP_X86_APIC_BUS_CYCLES_NS");
+        return -EOPNOTSUPP;
+    }
+
+    r = kvm_vm_enable_cap(kvm_state, KVM_CAP_X86_APIC_BUS_CYCLES_NS,
+                          0, TDX_APIC_BUS_CYCLES_NS);
+    if (r < 0) {
+        error_setg_errno(errp, -r,
+                         "Unable to set core crystal clock frequency to 25MHz");
+        return r;
+    }
+
 #define SHA384_DIGEST_SIZE  48
     if (tdx_guest->mrconfigid) {
         g_autofree uint8_t *data = qbase64_decode(tdx_guest->mrconfigid,
diff --git a/target/i386/kvm/tdx.h b/target/i386/kvm/tdx.h
index bc26e24eb9ac..0aebc7e3f6c9 100644
--- a/target/i386/kvm/tdx.h
+++ b/target/i386/kvm/tdx.h
@@ -14,6 +14,9 @@ typedef struct TdxGuestClass {
     X86ConfidentialGuestClass parent_class;
 } TdxGuestClass;
 
+/* TDX requires bus frequency 25MHz */
+#define TDX_APIC_BUS_CYCLES_NS 40
+
 typedef struct TdxGuest {
     X86ConfidentialGuest parent_obj;
 
-- 
2.34.1