On 4/1/2022 10:01 AM, Sean Christopherson wrote:
On Fri, Mar 04, 2022, Zeng Guang wrote:
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@xxxxxxxxxx>
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Signed-off-by: Zeng Guang <guang.zeng@xxxxxxxxx>
---
arch/x86/include/asm/kvm_host.h | 6 ++++++
arch/x86/kvm/x86.c | 11 +++++++++++
The new behavior needs to be documented in api.rst.
OK. I will prepare document for it.
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;
This should be max_vcpu_ids. I agree the it _should_ be max_vcpu_id, but KVM's API
for this is awful and we're stuck with the plural name.
};
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:
I think it makes sense to change kvm_vm_ioctl_check_extension() to return the
current max, it is a VM-scoped ioctl after all.
kvm_vm_ioctl_check_extension() can return kvm->arch.max_vcpu_ids
as it reflects runtime capability supported on current vm. I will
change it.
Amusingly, I think we also need a capability to enumerate that KVM_CAP_MAX_VCPU_ID
is writable.
IIUC, KVM_CAP_* has intrinsic writable attribute. KVM will return invalid
If not implemented.
+ if (cap->args[0] <= KVM_MAX_VCPU_IDS) {
+ kvm->arch.max_vcpu_id = cap->args[0];
This needs to be rejected if kvm->created_vcpus > 0, and that check needs to be
done under kvm_lock, otherwise userspace can bump the max ID after KVM allocates
per-VM structures and trigger buffer overflow.
Is it necessary to use kvm_lock ? Seems no use case to call it from multi-threads.
+ r = 0;
+ } else
If-elif-else statements need curly braces for all paths if any path needs braces.
Probably a moot point for this patch due to the above changes.
+ r = -E2BIG;
This should be -EINVAL, not -E2BIG.
E.g.
case KVM_CAP_MAX_VCPU_ID:
r = -EINVAL;
if (cap->args[0] > KVM_MAX_VCPU_IDS)
break;
mutex_lock(&kvm->lock);
if (!kvm->created_vcpus) {
kvm->arch.max_vcpu_id = cap->args[0];
r = 0;
}
mutex_unlock(&kvm->lock);
break;
+ 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;
Same here, it should be -EINVAL.
+
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
