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>

tests/integration.rs

@@ -1035,6 +1035,40 @@ fn test_cpu_topology(threads_per_core: u8, cores_per_package: u8, packages: u8,
                 .unwrap_or(0),
             packages
         );
+
+        #[cfg(target_arch = "x86_64")]
+        {
+            let mut cpu_id = 0;
+            for package_id in 0..packages {
+                for core_id in 0..cores_per_package {
+                    for _ in 0..threads_per_core {
+                        assert_eq!(
+                            guest
+                                .ssh_command(&format!("cat /sys/devices/system/cpu/cpu{cpu_id}/topology/physical_package_id"))
+                                .unwrap()
+                                .trim()
+                                .parse::<u8>()
+                                .unwrap_or(0),
+                            package_id
+                        );
+
+                        assert_eq!(
+                            guest
+                                .ssh_command(&format!(
+                                    "cat /sys/devices/system/cpu/cpu{cpu_id}/topology/core_id"
+                                ))
+                                .unwrap()
+                                .trim()
+                                .parse::<u8>()
+                                .unwrap_or(0),
+                            core_id
+                        );
+
+                        cpu_id += 1;
+                    }
+                }
+            }
+        }
     });
 
     let _ = child.kill();