[PATCH v2 5/7] KVM: Move check for existing vCPU ID to the top of vCPU creation

From: Sean Christopherson <seanjc@google.com>
To: Madhavan Srinivasan <maddy@linux.ibm.com>,
	Anup Patel <anup@brainfault.org>,  Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	 Sean Christopherson <seanjc@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	 Kiryl Shutsemau <kas@kernel.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: "Nicholas Piggin" <npiggin@gmail.com>,
	"Atish Patra" <atish.patra@linux.dev>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	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" <jean-christophe@guillain.net>,
	"Paweł S" <spawel523@gmail.com>
Subject: [PATCH v2 5/7] KVM: Move check for existing vCPU ID to the top of vCPU creation
Date: Mon, 21 Sep 2026 10:44:43 -0700	[thread overview]
Message-ID: <20260921174445.911676-6-seanjc@google.com> (raw)
In-Reply-To: <20260921174445.911676-1-seanjc@google.com>

Now that kvm->lock is held for the entirety of vCPU creation, check for a
conflicting vCPU ID at the begnning of vCPU creation, before the arch
precreate() hook is invoked.  This will allow reverting commit 97d65b544f48
("KVM: Check for duplicate vcpu_id as early as possible").  For now, keep
the redundant vcpu_ids tracking as a sanity check.

No functional change intended (absent KVM bugs, checking vcpu_ids and
walking kvm_get_vcpu_by_id() should yield the same result).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c17cc8dd371b..d5524ac8c5cf 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4182,7 +4182,10 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 	if (kvm->created_vcpus >= kvm->max_vcpus)
 		return -EINVAL;
 
-	if (test_bit(id, kvm->vcpu_ids))
+	if (kvm_get_vcpu_by_id(kvm, id))
+		return -EEXIST;
+
+	if (WARN_ON_ONCE(test_bit(id, kvm->vcpu_ids)))
 		return -EEXIST;
 
 	r = kvm_arch_vcpu_precreate(kvm, id);
@@ -4221,11 +4224,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 			goto arch_vcpu_destroy;
 	}
 
-	if (WARN_ON_ONCE(kvm_get_vcpu_by_id(kvm, id))) {
-		r = -EEXIST;
-		goto unlock_vcpu_destroy;
-	}
-
 	/*
 	 * Set the vCPU's index *before* the vCPU is reachable by other tasks.
 	 * Unwind the index back to -1 on failure so that KVM can use the index
-- 
2.55.0.1082.g2b9226bbc0-goog


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