platform: restrict SVSM request loop to SNP platforms

The SVSM request loop is not meaningful on non-SNP platforms, so it
should not be invoked on non-SNP platforms.

Signed-off-by: Jon Lange <jlange@microsoft.com>

kernel/src/cpu/percpu.rs

@@ -1365,14 +1365,17 @@ pub fn current_task() -> TaskPointer {
 
 #[no_mangle]
 pub extern "C" fn cpu_idle_loop() {
-    // Start request processing on this CPU.
-    let apic_id = this_cpu().get_apic_id();
-    let processing_name = format!("request-processing on CPU {}", apic_id);
-    start_kernel_task(request_processing_main, processing_name)
-        .expect("Failed to launch request processing task");
-    let loop_name = format!("request-loop on CPU {}", apic_id);
-    start_kernel_task(request_loop_main, loop_name)
-        .expect("Failed to launch request loop task");
+    // Start request processing on this CPU if required.
+    if SVSM_PLATFORM.start_svsm_request_loop() {
+        // Start request processing on this CPU.
+        let apic_id = this_cpu().get_apic_id();
+        let processing_name = format!("request-processing on CPU {}", apic_id);
+        start_kernel_task(request_processing_main, processing_name)
+            .expect("Failed to launch request processing task");
+        let loop_name = format!("request-loop on CPU {}", apic_id);
+        start_kernel_task(request_loop_main, loop_name)
+            .expect("Failed to launch request loop task");
+    }
 
     loop {
         // Go idle

kernel/src/platform/mod.rs

@@ -178,6 +178,11 @@ pub trait SvsmPlatform {
         cpu: &PerCpu,
         context: &hyperv::HvInitialVpContext,
     ) -> Result<(), SvsmError>;
+
+    /// Indicates whether this platform should invoke the SVSM request loop.
+    fn start_svsm_request_loop(&self) -> bool {
+        false
+    }
 }
 
 //FIXME - remove Copy trait

kernel/src/platform/snp.rs

@@ -330,6 +330,10 @@ impl SvsmPlatform for SnpPlatform {
 
         current_ghcb().ap_create(vmsa_pa, cpu.get_apic_id().into(), 0, sev_features)
     }
+
+    fn start_svsm_request_loop(&self) -> bool {
+        true
+    }
 }
 
 #[derive(Clone, Copy, Debug, Default)]