i386/tdx: Set APIC bus rate to match with what TDX module enforces

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 at 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>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-15-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/target/i386/kvm/tdx.c b/target/i386/kvm/tdx.c
index 671f23d910a7ae322a5d4d066b4853e08263fa2b..58983edd80ab73b6223835d359031c9588a2c83c 100644 (file)
--- a/target/i386/kvm/tdx.c
+++ b/target/i386/kvm/tdx.c
@@ -254,6 +254,19 @@ 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;
+    }
+
     if (tdx_guest->mrconfigid) {
         g_autofree uint8_t *data = qbase64_decode(tdx_guest->mrconfigid,
                               strlen(tdx_guest->mrconfigid), &data_len, errp);

diff --git a/target/i386/kvm/tdx.h b/target/i386/kvm/tdx.h
index e472b11fb0dd8fdcd73e79bb2e221ee66869e328..d39e733d9fcc4e48d95457800d3c32b932e2e83b 100644 (file)
--- a/target/i386/kvm/tdx.h
+++ b/target/i386/kvm/tdx.h
@@ -16,6 +16,9 @@ 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;