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Mon, 21 Sep 2026 10:44:51 -0700 (PDT) Date: Mon, 21 Sep 2026 10:44:42 -0700 In-Reply-To: <20260921174445.911676-1-seanjc@google.com> Mime-Version: 1.0 References: <20260921174445.911676-1-seanjc@google.com> X-Mailer: git-send-email 2.55.0.1082.g2b9226bbc0-goog Message-ID: <20260921174445.911676-5-seanjc@google.com> Subject: [PATCH v2 4/7] KVM: Protect all of kvm_vm_ioctl_create_vcpu() with kvm->lock From: Sean Christopherson To: Madhavan Srinivasan , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Sean Christopherson , Paolo Bonzini , Kiryl Shutsemau , Rick Edgecombe Cc: Nicholas Piggin , Atish Patra , Alexandre Ghiti , Dave Hansen , linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, Jean-Christophe Guillain , "=?UTF-8?q?Pawe=C5=82=20S?=" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260921_184454_319354_B7786A40 X-CRM114-Status: GOOD ( 16.07 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Sean Christopherson Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org 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 --- 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 0409ac9e7b31..30f7095a156e 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 eca4a4359ab2..cc628afbb850 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 78cc090435be..c17cc8dd371b 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -4165,6 +4165,8 @@ 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. @@ -4177,26 +4179,18 @@ 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) { @@ -4227,8 +4221,6 @@ 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; @@ -4267,7 +4259,6 @@ 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; @@ -4278,7 +4269,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) 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); @@ -4287,10 +4277,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) 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; } -- 2.55.0.1082.g2b9226bbc0-goog _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv