From 6418b715d5a24fa952e8c4baa587dd812dd4765b Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Mon, 21 Sep 2026 10:44:42 -0700 Subject: [PATCH] KVM: Protect all of kvm_vm_ioctl_create_vcpu() with kvm->lock When creating a vCPU, don't drop kvm->lock to when doing the bulk of actual vCPU creation, as allowing multiple vCPUs to be created in parallel adds significant complexity in KVM (as evidenced by the many related bugs), and all known VMMs fully serialize vCPU creation. Remove all manually locking of kvm->lock from kvm_arch_vcpu_{post,}create() for obvious reasons. For many years, "everyone" has assumed that dropping kvm->lock was done for performance reasons optimization, e.g. to allow userspace to create all vCPUs concurrently for latency purposes. But as above, no known VMM does that. Looking at the history of this code, before commit 11ec28047118 ("KVM: Convert vm lock to a mutex"), kvm->lock was a spinlock. I.e. KVM *had* to drop kvm->lock when doing the bulk of vCPU creation, otherwise KVM couldn't do normal memory allocations. When kvm->lock got turned into a mutex for unrelated reasons, no one took advantage updated of the change to simplify vCPU creation. And 19 years later, everyone just assumed that KVM continued to deal with the complexity for performance reasons. Furthermore, naively parallelizing vCPU creation in userspace is likely a net negative due to the overheads of task creation. Unless a VMM carefully avoids the extra overhead related to parallelization, e.g. spawns each vCPU's thread before creating the vCPU, creating vCPUs concurrently is a net *negative* up until about ~64 vCPUs, after which the times are a wash. The absolute speed of light _is_ faster if KVM doesn't hold kvm-lock, but at vCPU counts of ~16 or less, it's probably in the noise when considering total VM creation time, as the added latency is less than 1ms up until 16 or so vCPUs. On top of all that, KVM has had a *lot* of fatal bugs (most often found by syzkaller) related to vCPUs being created while trying to do per-VM operations (basically, see every flow that locks all vCPUs). I.e. the parallel vCPU creation "support" is actively harmful as the only "use case" is for misbehaving userspace to exploit KVM bugs. Serializing vCPU creation will allow reverting commit 97d65b544f48 ("KVM: Check for duplicate vcpu_id as early as possible"), which had "minor" math error: the worst case scenario isn't "256 bytes per VM", it's "256 unsigned longs per VM", i.e. 2048 bytes per VM, which doubles the size of each VM and pushes several architectures into order-1 allocations. Signed-off-by: Sean Christopherson Tested-by: Jean-Christophe Guillain Tested-by: Naveen N Rao (AMD) Message-ID: <20260921174445.911676-5-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/powerpc/kvm/book3s_hv.c | 2 -- arch/s390/kvm/s390/s390.c | 5 +---- virt/kvm/kvm_main.c | 22 +++++----------------- 3 files changed, 6 insertions(+), 23 deletions(-) diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c index dbac3573b2c8..de05d721edc4 100644 --- a/arch/powerpc/kvm/book3s_hv.c +++ b/arch/powerpc/kvm/book3s_hv.c @@ -3058,7 +3058,6 @@ static int kvmppc_core_vcpu_create_hv(struct kvm_vcpu *vcpu) init_waitqueue_head(&vcpu->arch.cpu_run); - mutex_lock(&kvm->lock); vcore = NULL; err = -EINVAL; if (cpu_has_feature(CPU_FTR_ARCH_300)) { @@ -3091,7 +3090,6 @@ static int kvmppc_core_vcpu_create_hv(struct kvm_vcpu *vcpu) mutex_unlock(&kvm->arch.mmu_setup_lock); } } - mutex_unlock(&kvm->lock); if (!vcore) return err; diff --git a/arch/s390/kvm/s390/s390.c b/arch/s390/kvm/s390/s390.c index efd77b9cd36c..e4912db67adc 100644 --- a/arch/s390/kvm/s390/s390.c +++ b/arch/s390/kvm/s390/s390.c @@ -3579,12 +3579,11 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu) { - mutex_lock(&vcpu->kvm->lock); preempt_disable(); vcpu->arch.sie_block->epoch = vcpu->kvm->arch.epoch; vcpu->arch.sie_block->epdx = vcpu->kvm->arch.epdx; preempt_enable(); - mutex_unlock(&vcpu->kvm->lock); + if (!kvm_is_ucontrol(vcpu->kvm)) { vcpu->arch.gmap = vcpu->kvm->arch.gmap; sca_add_vcpu(vcpu); @@ -3757,13 +3756,11 @@ static int kvm_s390_vcpu_setup(struct kvm_vcpu *vcpu) kvm_s390_vcpu_pci_setup(vcpu); - mutex_lock(&vcpu->kvm->lock); if (kvm_s390_pv_is_protected(vcpu->kvm)) { rc = kvm_s390_pv_create_cpu(vcpu, &uvrc, &uvrrc); if (rc) kvm_s390_vcpu_unsetup_cmma(vcpu); } - mutex_unlock(&vcpu->kvm->lock); return rc; } diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index aad8cf00a1c8..cae0a9cdc33e 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -4188,6 +4188,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) struct kvm_vcpu *vcpu; struct page *page; + guard(mutex)(&kvm->lock); + /* * KVM tracks vCPU IDs as 'int', be kind to userspace and reject * too-large values instead of silently truncating. @@ -4200,26 +4202,18 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) if (id >= KVM_MAX_VCPU_IDS) return -EINVAL; - mutex_lock(&kvm->lock); - if (kvm->created_vcpus >= kvm->max_vcpus) { - mutex_unlock(&kvm->lock); + if (kvm->created_vcpus >= kvm->max_vcpus) return -EINVAL; - } - if (test_bit(id, kvm->vcpu_ids)) { - mutex_unlock(&kvm->lock); + if (test_bit(id, kvm->vcpu_ids)) return -EEXIST; - } r = kvm_arch_vcpu_precreate(kvm, id); - if (r) { - mutex_unlock(&kvm->lock); + if (r) return r; - } kvm->created_vcpus++; __set_bit(id, kvm->vcpu_ids); - mutex_unlock(&kvm->lock); vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL_ACCOUNT); if (!vcpu) { @@ -4250,8 +4244,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) goto arch_vcpu_destroy; } - mutex_lock(&kvm->lock); - if (WARN_ON_ONCE(kvm_get_vcpu_by_id(kvm, id))) { r = -EEXIST; goto unlock_vcpu_destroy; @@ -4290,7 +4282,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) atomic_inc(&kvm->online_vcpus); mutex_unlock(&vcpu->mutex); - mutex_unlock(&kvm->lock); kvm_arch_vcpu_postcreate(vcpu); kvm_create_vcpu_debugfs(vcpu); return r; @@ -4301,7 +4292,6 @@ kvm_put_xa_erase: xa_erase(&kvm->vcpu_array, vcpu->vcpu_idx); unlock_vcpu_destroy: vcpu->vcpu_idx = -1; - mutex_unlock(&kvm->lock); kvm_dirty_ring_free(&vcpu->dirty_ring); arch_vcpu_destroy: kvm_arch_vcpu_destroy(vcpu); @@ -4310,10 +4300,8 @@ vcpu_free_run_page: vcpu_free: kmem_cache_free(kvm_vcpu_cache, vcpu); vcpu_decrement: - mutex_lock(&kvm->lock); kvm->created_vcpus--; __clear_bit(id, kvm->vcpu_ids); - mutex_unlock(&kvm->lock); return r; }