tdx: normalize TDX max_phys_bits handling

arch/src/x86_64/mod.rs

@@ -631,8 +631,6 @@ pub fn generate_common_cpuid(
 
     CpuidPatch::patch_cpuid(&mut cpuid, &cpuid_patches);
 
-    let la57_enabled = CpuidPatch::is_feature_enabled(&cpuid, 7, 0, CpuidReg::ECX, 16);
-
     #[cfg(feature = "tdx")]
     let tdx_capabilities = if config.tdx {
         let tdx_vm = vm.ok_or_else(|| {
@@ -715,14 +713,9 @@ pub fn generate_common_cpuid(
             }
             // Set CPU physical bits
             0x8000_0008 => {
-                let virt_addr_width = if la57_enabled { 57 } else { 48 };
-                let mut guest_phys_bits = (entry.eax >> 16) & 0xff;
-                if guest_phys_bits > config.phys_bits as u32 {
-                    guest_phys_bits = config.phys_bits as u32;
-                }
-                entry.eax = (config.phys_bits as u32 & 0xff)
-                    | (virt_addr_width << 8)
-                    | (guest_phys_bits << 16);
+                entry.eax = (entry.eax & 0xff00_ff00)
+                    | (config.phys_bits as u32 & 0xff)
+                    | ((config.phys_bits as u32 & 0xff) << 16);
             }
             0x4000_0001 => {
                 // Enable KVM_FEATURE_MSI_EXT_DEST_ID. This allows the guest to target

docs/intel_tdx.md

@@ -70,7 +70,7 @@ guest image containing the TDX enlightened kernel.
 ./cloud-hypervisor \
     --platform tdx=on \
     --firmware edk2/Build/IntelTdx/RELEASE_GCC5/FV/OVMF.fd \
-    --cpus boot=1,max_phys_bits=52 \
+    --cpus boot=1 \
     --memory size=1G \
     --serial tty \
     --console off \
@@ -84,7 +84,7 @@ firmware:
 ./cloud-hypervisor \
     --platform tdx=on \
     --firmware edk2/Build/IntelTdx/DEBUG_GCC5/FV/OVMF.fd \
-    --cpus boot=1,max_phys_bits=52 \
+    --cpus boot=1 \
     --memory size=1G \
     --disk path=tdx_guest_img \
     --serial tty \
@@ -139,7 +139,7 @@ option as well.
     --firmware td-shim/target/release/final.bin \
     --kernel bzImage \
     --cmdline "root=/dev/vda1 rw console=ttyS0 ignore_loglevel earlyprintk=ttyS0" \
-    --cpus boot=1,max_phys_bits=52  \
+    --cpus boot=1  \
     --memory size=1G \
     --disk path=tdx_guest_img
 ```
@@ -153,7 +153,7 @@ TDShim:
     --firmware td-shim/target/debug/final.bin \
     --kernel bzImage \
     --cmdline "root=/dev/vda1 rw console=ttyS0 ignore_loglevel earlyprintk=ttyS0" \
-    --cpus boot=1,max_phys_bits=52  \
+    --cpus boot=1  \
     --memory size=1G \
     --disk path=tdx_guest_img
 ```

vmm/src/config.rs

@@ -2873,6 +2873,16 @@ impl VmConfig {
             if tdx_enabled && (self.cpus.max_vcpus != self.cpus.boot_vcpus) {
                 return Err(ValidationError::TdxNoCpuHotplug);
             }
+            if tdx_enabled {
+                // For TDX the guest physical address width (GPAW) is fixed at:
+                // 48 - (GPAW-48, 4-level EPT)
+                // 52 - (GPAW-52, 5-level EPT)
+                if self.cpus.max_phys_bits < 52 {
+                    self.cpus.max_phys_bits = 48;
+                } else {
+                    self.cpus.max_phys_bits = 52;
+                }
+            }
         }
 
         #[cfg(feature = "sev_snp")]
@@ -4898,6 +4908,26 @@ id=\"{id}\",pci_segment={pci_segment},queue_sizes={queue_sizes}"
 
         valid_config.validate().unwrap();
 
+        #[cfg(feature = "tdx")]
+        {
+            let mut tdx_config = valid_config.clone();
+            tdx_config.cpus.max_phys_bits = 51;
+            tdx_config.platform = Some(PlatformConfig {
+                tdx: true,
+                ..platform_fixture()
+            });
+            if let Some(payload) = tdx_config.payload.as_mut() {
+                payload.firmware = Some(PathBuf::from("/path/to/firmware"));
+            }
+
+            tdx_config.validate().unwrap();
+            assert_eq!(tdx_config.cpus.max_phys_bits, 48);
+
+            tdx_config.cpus.max_phys_bits = 52;
+            tdx_config.validate().unwrap();
+            assert_eq!(tdx_config.cpus.max_phys_bits, 52);
+        }
+
         let mut invalid_config = valid_config.clone();
         invalid_config.serial.mode = ConsoleOutputMode::Tty;
         invalid_config.console.mode = ConsoleOutputMode::Tty;