kvm: x86: Set KVM_MAX_VCPU_ID to 4*KVM_MAX_VCPUS

Instead of requiring KVM_MAX_VCPU_ID to be manually increased
every time we increase KVM_MAX_VCPUS, set it to 4*KVM_MAX_VCPUS.
This should be enough for CPU topologies where Cores-per-Package
and Packages-per-Socket are not powers of 2.

In practice, this increases KVM_MAX_VCPU_ID from 1023 to 1152.
The only side effect of this change is making some fields in
struct kvm_ioapic larger, increasing the struct size from 1628 to
1780 bytes (in x86_64).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20210903211600.2002377-2-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -39,7 +39,19 @@
 
 #define KVM_MAX_VCPUS 288
 #define KVM_SOFT_MAX_VCPUS 240
-#define KVM_MAX_VCPU_ID 1023
+
+/*
+ * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs
+ * might be larger than the actual number of VCPUs because the
+ * APIC ID encodes CPU topology information.
+ *
+ * In the worst case, we'll need less than one extra bit for the
+ * Core ID, and less than one extra bit for the Package (Die) ID,
+ * so ratio of 4 should be enough.
+ */
+#define KVM_VCPU_ID_RATIO 4
+#define KVM_MAX_VCPU_ID (KVM_MAX_VCPUS * KVM_VCPU_ID_RATIO)
+
 /* memory slots that are not exposed to userspace */
 #define KVM_PRIVATE_MEM_SLOTS 3