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KVM: Initialize a vCPU's index to '-1' while it's being created

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Invalidate a vCPU's index immediately after allocating storage for the vCPU
so that KVM doesn't incorrectly treat a vCPU that is the process of being
created as being vCPU0.  This will also allow detecting that a vCPU is in
the process of being created and thus otherwise unreachable, which is
useful for avoiding false positives in lockdep assertions on vcpu->mutex.

Unwind the index back to -1 if inserting the vCPU into the array or adding
the vCPU to the fd table fails, so that kvm_arch_vcpu_destroy() sees the
vCPU as unreachable, i.e. so that teardown logic doesn't hit false positive
lockdep assertions.  Opportunistically add a comment to call out that the
"real" index needs to be set before making the vCPU visible to other tasks.

Note, kvm_wait_for_vcpu_online() naturally does the right thing thanks to
vcpu->vcpu_idx and kvm->online_vcpus being signed values.

Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Link: https://patch.msgid.link/20260630225619.511632-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2026-06-30 15:56:14 -07:00
parent c10bd49bdc
commit b29125ead0
1 changed files with 8 additions and 0 deletions
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8
virt/kvm/kvm_main.c
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@ -4188,6 +4188,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
goto vcpu_decrement;
}
vcpu->vcpu_idx = -1;
BUILD_BUG_ON(sizeof(struct kvm_run) > PAGE_SIZE);
page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
if (!page) {
@ -4216,6 +4218,11 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
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
* to detect that the vCPU is unreachable, e.g. for lockdep asserts.
*/
vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
WARN_ON_ONCE(r == -EBUSY);
@ -4254,6 +4261,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
kvm_put_kvm_no_destroy(kvm);
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: