vmm: skip configure_system when rsdp_addr is None

For SEV-SNP guests using IGVM, the ACPI tables and system
configuration (MP tables, EBDA, SMBIOS, PVH start info, e820)
are provided by the IGVM file. The rsdp_addr is set to None
for these guests to indicate ACPI table creation was skipped.

Commit 7d65187350 ("vmm: make RSDP address optional in
configure_system") removed the guard that prevented calling
configure_system when rsdp_addr is None. This caused MSHV
SEV-SNP guests to crash because configure_system writes to
guest memory locations that conflict with the IGVM-provided
layout.

Restore the guard by only calling configure_system when
rsdp_addr is Some, which preserves the intended behavior
for CVM guests while still allowing the Option<GuestAddress>
refactoring.

Assisted-by: Claude:Opus-4.6

Signed-off-by: Muminul Islam <muislam@microsoft.com>

vmm/src/vm.rs

@@ -2840,10 +2840,13 @@ impl Vm {
         let rsdp_addr = self.create_acpi_tables();
 
         #[cfg(not(target_arch = "riscv64"))]
-        // Configure shared state based on loaded kernel
-        entry_point
-            .map(|entry_point| self.configure_system(rsdp_addr, entry_point))
-            .transpose()?;
+        // Configure shared state based on loaded kernel.
+        // Skip for SEV-SNP guests where system configuration is provided via IGVM.
+        if rsdp_addr.is_some() {
+            entry_point
+                .map(|entry_point| self.configure_system(rsdp_addr, entry_point))
+                .transpose()?;
+        }
 
         #[cfg(target_arch = "riscv64")]
         self.configure_system().unwrap();