Re: [PATCH 04/13] KVM: WARN if there are danging MMU invalidations at VM destruction

[Binbin Wu]

On 9/22/2023 4:33 AM, Sean Christopherson wrote:
> Add an assertion that there are no in-progress MMU invalidations when a
> VM is being destroyed, with the exception of the scenario where KVM
> unregisters its MMU notifier between an .invalidate_range_start() call and
> the corresponding .invalidate_range_end().
>
> KVM can't detect unpaired calls from the mmu_notifier due to the above
> exception waiver, but the assertion can detect KVM bugs, e.g. such as the
> bug that *almost* escaped initial guest_memfd development.
>
> Link: https://lore.kernel.org/all/e397d30c-c6af-e68f-d18e-b4e3739c5389@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   virt/kvm/kvm_main.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 54480655bcce..277afeedd670 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1381,9 +1381,16 @@ static void kvm_destroy_vm(struct kvm *kvm)
>   	 * No threads can be waiting in kvm_swap_active_memslots() as the
>   	 * last reference on KVM has been dropped, but freeing
>   	 * memslots would deadlock without this manual intervention.
> +	 *
> +	 * If the count isn't unbalanced, i.e. KVM did NOT unregister between
Nit: Readers can get it according to the code context, but is it better 
to add
"MMU notifier"  to tell what to "unregister" to make the comment easier to
understand?


> +	 * a start() and end(), then there shouldn't be any in-progress
> +	 * invalidations.
>   	 */
>   	WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait));
> -	kvm->mn_active_invalidate_count = 0;
> +	if (kvm->mn_active_invalidate_count)
> +		kvm->mn_active_invalidate_count = 0;
> +	else
> +		WARN_ON(kvm->mmu_invalidate_in_progress);
>   #else
>   	kvm_flush_shadow_all(kvm);
>   #endif