From: Xiaoyao Li <xiaoyao.li@intel.com>

On Intel CPU, the value of CPUID_EXT2_SYSCALL depends on the mode of
the vcpu. It's 0 outside 64-bit mode and 1 in 64-bit mode.

The initial state of TDX vcpu is 32-bit protected mode. At the time of
calling KVM_TDX_GET_CPUID, vcpu hasn't started running so the value read
is 0.

In reality, 64-bit mode should always be supported. So mark
CPUID_EXT2_SYSCALL always supported to avoid false warning.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-53-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/kvm/tdx.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/target/i386/kvm/tdx.c b/target/i386/kvm/tdx.c
index e474abf3a60..76293029917 100644
--- a/target/i386/kvm/tdx.c
+++ b/target/i386/kvm/tdx.c
@@ -845,6 +845,19 @@ static int tdx_check_features(X86ConfidentialGuest *cg, CPUState *cs)
             continue;
         }
 
+        /* Fixup for special cases */
+        switch (w) {
+        case FEAT_8000_0001_EDX:
+            /*
+             * Intel enumerates SYSCALL bit as 1 only when processor in 64-bit
+             * mode and before vcpu running it's not in 64-bit mode.
+             */
+            actual |= CPUID_EXT2_SYSCALL;
+            break;
+        default:
+            break;
+        }
+
         requested = env->features[w];
         unavailable = requested & ~actual;
         mark_unavailable_features(cpu, w, unavailable, unav_prefix);
-- 
2.49.0