[PATCH v6 8/9] KVM: x86: Allow userspace set maximum VCPU id for VM

Zeng Guang
2022-02-25T08:54:06Z

Introduce new max_vcpu_id in KVM for x86 architecture. Userspace
can assign maximum possible vcpu id for current VM session using
KVM_CAP_MAX_VCPU_ID of KVM_ENABLE_CAP ioctl().

This is done for x86 only because the sole use case is to guide
memory allocation for PID-pointer table, a structure needed to
enable VMX IPI.

By default, max_vcpu_id set as KVM_MAX_VCPU_IDS.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Zeng Guang <guang.zeng@intel.com>
---
No new KVM capability is added to advertise the support of
configurable maximum vCPU ID to user space because max_vcpu_id is
just a hint/commitment to allow KVM to reduce the size of PID-pointer
table. But I am not 100% sure if it is proper to do so.

arch/x86/include/asm/kvm_host.h | 6 ++++++
arch/x86/kvm/x86.c | 11 +++++++++++
2 files changed, 17 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6dcccb304775..db16aebd946c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1233,6 +1233,12 @@ struct kvm_arch {
hpa_t hv_root_tdp;
spinlock_t hv_root_tdp_lock;
#endif
+ /*
+ * VM-scope maximum vCPU ID. Used to determine the size of structures
+ * that increase along with the maximum vCPU ID, in which case, using
+ * the global KVM_MAX_VCPU_IDS may lead to significant memory waste.
+ */
+ u32 max_vcpu_id;
};

struct kvm_vm_stat {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4f6fe9974cb5..ca17cc452bd3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5994,6 +5994,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
kvm->arch.exit_on_emulation_error = cap->args[0];
r = 0;
break;
+ case KVM_CAP_MAX_VCPU_ID:
+ if (cap->args[0] <= KVM_MAX_VCPU_IDS) {
+ kvm->arch.max_vcpu_id = cap->args[0];
+ r = 0;
+ } else
+ r = -E2BIG;
+ break;
default:
r = -EINVAL;
break;
@@ -11067,6 +11074,9 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
struct page *page;
int r;

+ if (vcpu->vcpu_id >= vcpu->kvm->arch.max_vcpu_id)
+ return -E2BIG;
+
vcpu->arch.last_vmentry_cpu = -1;
vcpu->arch.regs_avail = ~0;
vcpu->arch.regs_dirty = ~0;
@@ -11589,6 +11599,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
spin_lock_init(&kvm->arch.hv_root_tdp_lock);
kvm->arch.hv_root_tdp = INVALID_PAGE;
#endif
+ kvm->arch.max_vcpu_id = KVM_MAX_VCPU_IDS;

INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
INIT_DELAYED_WORK(&kvm->arch.kvmclock_sync_work, kvmclock_sync_fn);
--
2.27.0