[QEMU PATCH v2] x86: Set maximum APIC ID to KVM prior to vCPU creation

[QEMU PATCH v2] x86: Set maximum APIC ID to KVM prior to vCPU creation

[Zeng Guang]

Specify maximum possible APIC ID assigned for current VM session to KVM
prior to the creation of vCPUs. By this setting, KVM can set up VM-scoped
data structure indexed by the APIC ID, e.g. Posted-Interrupt Descriptor
table to support Intel IPI virtualization, with the most optimal memory
footprint.

It can be achieved by calling KVM_ENABLE_CAP for KVM_CAP_MAX_VCPU_ID
capability once KVM has already enabled it. Ignoring the return error
if KVM doesn't support this capability yet.

Signed-off-by: Zeng Guang <guang.zeng@intel.com>
---
 hw/i386/x86.c              | 5 +++++
 target/i386/kvm/kvm.c      | 5 +++++
 target/i386/kvm/kvm_i386.h | 1 +
 3 files changed, 11 insertions(+)

diff --git a/hw/i386/x86.c b/hw/i386/x86.c
index 4cf107baea..a6ab406f85 100644
--- a/hw/i386/x86.c
+++ b/hw/i386/x86.c
@@ -123,6 +123,11 @@ void x86_cpus_init(X86MachineState *x86ms, int default_cpu_version)
      */
     x86ms->apic_id_limit = x86_cpu_apic_id_from_index(x86ms,
                                                       ms->smp.max_cpus - 1) + 1;
+
+    if (kvm_enabled()) {
+        kvm_set_max_apic_id(x86ms->apic_id_limit);
+    }
+
     possible_cpus = mc->possible_cpu_arch_ids(ms);
     for (i = 0; i < ms->smp.cpus; i++) {
         x86_cpu_new(x86ms, possible_cpus->cpus[i].arch_id, &error_fatal);
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 9cf8e03669..cb2fe39365 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -5251,3 +5251,8 @@ void kvm_request_xsave_components(X86CPU *cpu, uint64_t mask)
         mask &= ~BIT_ULL(bit);
     }
 }
+
+void kvm_set_max_apic_id(uint32_t max_apic_id)
+{
+    kvm_vm_enable_cap(kvm_state, KVM_CAP_MAX_VCPU_ID, 0, max_apic_id);
+}
diff --git a/target/i386/kvm/kvm_i386.h b/target/i386/kvm/kvm_i386.h
index 4124912c20..c133b32a58 100644
--- a/target/i386/kvm/kvm_i386.h
+++ b/target/i386/kvm/kvm_i386.h
@@ -54,4 +54,5 @@ uint64_t kvm_swizzle_msi_ext_dest_id(uint64_t address);
 bool kvm_enable_sgx_provisioning(KVMState *s);
 void kvm_request_xsave_components(X86CPU *cpu, uint64_t mask);
 
+void kvm_set_max_apic_id(uint32_t max_apic_id);
 #endif
-- 
2.27.0