arch: x86_64: handle npot CPU topology

This PR addresses a bug in which the cpu topology of a guest
with non power-of-two number of cores is incorrect. For example,
in some contexts, a virtual machine with 2-sockets and 12-cores
will incorrectly believe that 16 cores are on socket 1 and 8
cores are on socket 2. In other cases, common topology enumeration
software such as hwloc will crash.

The root of the problem was the way that cloud-hypervisor generates
apic_id. On x86_64, the (x2) apic_id embeds information about cpu
topology. The cpuid instruction is primarily used to discover the
number of sockets, dies, cores, threads, etc. Using this information,
the (x2) apic_id is masked to determine which {core, die, socket} the
cpu is on. When the cpu topology is not a power of two
(e.g. a 12-core machine), this requires non-contiguous (x2) apic_id.

Signed-off-by: Thomas Barrett <tbarrett@crusoeenergy.com>

vmm/src/vm.rs

@@ -1175,6 +1175,8 @@ impl Vm {
             .as_deref()
             .map(|strings| strings.iter().map(|s| s.as_ref()).collect::<Vec<&str>>());
 
+        let topology = self.cpu_manager.lock().unwrap().get_vcpu_topology();
+
         arch::configure_system(
             &mem,
             arch::layout::CMDLINE_START,
@@ -1185,6 +1187,7 @@ impl Vm {
             serial_number.as_deref(),
             uuid.as_deref(),
             oem_strings.as_deref(),
+            topology,
         )
         .map_err(Error::ConfigureSystem)?;
         Ok(())