vmm: Parse guest policy from IGVM initialization headers

Extract the SNP guest policy from IGVM initialization headers when
available, falling back to the default policy. This matches QEMU's
behaviour where only a non-zero IGVM policy overrides the default.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>

vmm/src/igvm/igvm_loader.rs

@@ -8,6 +8,8 @@ use std::mem::size_of;
 use std::sync::{Arc, Mutex};
 
 use hypervisor::HypervisorType;
+#[cfg(feature = "sev_snp")]
+use igvm::IgvmInitializationHeader;
 use igvm::snp_defs::SevVmsa;
 use igvm::{IgvmDirectiveHeader, IgvmFile, IgvmPlatformHeader};
 #[cfg(feature = "sev_snp")]
@@ -229,6 +231,20 @@ fn import_parameter(
     Ok(())
 }
 
+///
+/// Extract guest policy from the IGVM initialization headers.
+#[cfg(feature = "sev_snp")]
+pub fn extract_guest_policy(igvm_file: &IgvmFile) -> Option<igvm_defs::SnpPolicy> {
+    for header in igvm_file.initializations() {
+        if let IgvmInitializationHeader::GuestPolicy { policy, .. } = header
+            && *policy != 0
+        {
+            return Some(igvm_defs::SnpPolicy::from_bits(*policy));
+        }
+    }
+    None
+}
+
 ///
 /// Extract sev_features from the boot CPU (vp_index 0) VMSA.
 ///

vmm/src/vm.rs

@@ -1012,8 +1012,16 @@ impl Vm {
             .create_boot_vcpus(snapshot_from_id(snapshot, CPU_MANAGER_SNAPSHOT_ID))
             .map_err(Error::CpuManager)?;
 
-        // Initialize SEV-SNP - transitions guest into secure state
-        vm.sev_snp_init(Self::get_default_sev_snp_guest_policy())
+        // Extract guest policy from IGVM if available, otherwise use default.
+        #[cfg(feature = "igvm")]
+        let guest_policy = igvm_file
+            .as_ref()
+            .and_then(igvm_loader::extract_guest_policy)
+            .unwrap_or_else(Self::get_default_sev_snp_guest_policy);
+        #[cfg(not(feature = "igvm"))]
+        let guest_policy = Self::get_default_sev_snp_guest_policy();
+
+        vm.sev_snp_init(guest_policy)
             .map_err(Error::InitializeSevSnpVm)?;
 
         // Load payload for SEV-SNP (IGVM parser needs cpu_manager for cpuid)