i386/tdx: Validate phys_bits against host value

For TDX guest, the phys_bits is not configurable and can only be
host/native value.

Validate phys_bits inside tdx_check_features().

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/20250508150002.689633-55-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/host-cpu.c

@@ -15,7 +15,7 @@
 #include "system/system.h"
 
 /* Note: Only safe for use on x86(-64) hosts */
-static uint32_t host_cpu_phys_bits(void)
+uint32_t host_cpu_phys_bits(void)
 {
     uint32_t eax;
     uint32_t host_phys_bits;

target/i386/host-cpu.h

@@ -10,6 +10,7 @@
 #ifndef HOST_CPU_H
 #define HOST_CPU_H
 
+uint32_t host_cpu_phys_bits(void);
 void host_cpu_instance_init(X86CPU *cpu);
 void host_cpu_max_instance_init(X86CPU *cpu);
 bool host_cpu_realizefn(CPUState *cs, Error **errp);

target/i386/kvm/tdx.c

@@ -25,6 +25,7 @@
 
 #include "cpu.h"
 #include "cpu-internal.h"
+#include "host-cpu.h"
 #include "hw/i386/e820_memory_layout.h"
 #include "hw/i386/tdvf.h"
 #include "hw/i386/x86.h"
@@ -879,6 +880,13 @@ static int tdx_check_features(X86ConfidentialGuest *cg, CPUState *cs)
         return -EINVAL;
     }
 
+    if (cpu->phys_bits != host_cpu_phys_bits()) {
+        error_report("TDX requires guest CPU physical bits (%u) "
+                     "to match host CPU physical bits (%u)",
+                     cpu->phys_bits, host_cpu_phys_bits());
+        return -EINVAL;
+    }
+
     return 0;
 }