vmm: raise the (v)CPU limit on kvm/x86_64

Raise the max number of supported (v)CPUs on kvm x86_64 hosts
to 8192 (the max allowed value of CONFIG_NR_CPUS in the Linux kernel).

Other platfroms keep their existing CPU limits pending further
development and testing.

The change has been tested on Intel and AMD hosts.

Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Neel Natu <neelnatu@google.com>
Signed-off-by: Ofir Weisse <oweisse@google.com>
Signed-off-by: Peter Oskolkov <posk@google.com>

vmm/src/lib.rs

@@ -27,6 +27,8 @@ use anyhow::anyhow;
 #[cfg(feature = "dbus_api")]
 use api::dbus::{DBusApiOptions, DBusApiShutdownChannels};
 use api::http::HttpApiHandle;
+#[cfg(all(feature = "kvm", target_arch = "x86_64"))]
+use arch::x86_64::MAX_SUPPORTED_CPUS_LEGACY;
 use console_devices::{pre_create_console_devices, ConsoleInfo};
 use landlock::LandlockError;
 use libc::{tcsetattr, termios, EFD_NONBLOCK, SIGINT, SIGTERM, TCSANOW};
@@ -888,6 +890,11 @@ impl Vmm {
             ))
         })?;
 
+        #[cfg(all(feature = "kvm", target_arch = "x86_64"))]
+        if config.lock().unwrap().max_apic_id() > MAX_SUPPORTED_CPUS_LEGACY {
+            vm.enable_x2apic_api().unwrap();
+        }
+
         let phys_bits =
             vm::physical_bits(&self.hypervisor, config.lock().unwrap().cpus.max_phys_bits);
 
@@ -1822,7 +1829,7 @@ impl RequestHandler for Vmm {
         } else {
             let mut config = self.vm_config.as_ref().unwrap().lock().unwrap();
             if let Some(desired_vcpus) = desired_vcpus {
-                config.cpus.boot_vcpus = desired_vcpus.try_into().unwrap();
+                config.cpus.boot_vcpus = desired_vcpus;
             }
             if let Some(desired_ram) = desired_ram {
                 config.memory.size = desired_ram;