Re: [PATCH v3 08/12] KVM: x86/mmu: Split kvm_mmu_zap_all_fast() into "front" and "back" halves

[Michael Roth]

On Tue, Jun 30, 2026 at 03:26:03PM -0700, Sean Christopherson wrote:
> Split kvm_mmu_zap_all_fast() into a "front half" and a "back half", where
> the front half is everything that runs with mmu_lock held for write, and
> the back half is the code that runs outside of mmu_lock.  This will allow
> putting more code inside kvm_arch_flush_shadow_memslot()'s critical section
> without having to take mmu_lock twice in quick succession.
> 
> No functional change intended.
> 
> Cc: stable@xxxxxxxxxxxxxxx # 6.12.x
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Michael Roth <michael.roth@xxxxxxx>

> ---
>  arch/x86/kvm/mmu/mmu.c | 37 +++++++++++++++++++++++++------------
>  1 file changed, 25 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 223d80b12b9b..a5c2a560a88a 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6921,20 +6921,11 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
>  	kvm_mmu_commit_zap_page(kvm, &invalid_list);
>  }
>  
> -/*
> - * Fast invalidate all shadow pages and use lock-break technique
> - * to zap obsolete pages.
> - *
> - * It's required when memslot is being deleted or VM is being
> - * destroyed, in these cases, we should ensure that KVM MMU does
> - * not use any resource of the being-deleted slot or all slots
> - * after calling the function.
> - */
> -static void kvm_mmu_zap_all_fast(struct kvm *kvm)
> +static void __kvm_mmu_zap_all_fast_front_half(struct kvm *kvm)
>  {
>  	lockdep_assert_held(&kvm->slots_lock);
> +	lockdep_assert_held_write(&kvm->mmu_lock);
>  
> -	write_lock(&kvm->mmu_lock);
>  	trace_kvm_mmu_zap_all_fast(kvm);
>  
>  	/*
> @@ -6971,8 +6962,12 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
>  	kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_FREE_OBSOLETE_ROOTS);
>  
>  	kvm_zap_obsolete_pages(kvm);
> +}
>  
> -	write_unlock(&kvm->mmu_lock);
> +static void __kvm_mmu_zap_all_fast_back_half(struct kvm *kvm)
> +{
> +	lockdep_assert_held(&kvm->slots_lock);
> +	lockdep_assert_not_held(&kvm->mmu_lock);
>  
>  	/*
>  	 * Zap the invalidated TDP MMU roots, all SPTEs must be dropped before
> @@ -6986,6 +6981,24 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
>  		kvm_tdp_mmu_zap_invalidated_roots(kvm, true);
>  }
>  
> +/*
> + * Fast invalidate all shadow pages and use lock-break technique
> + * to zap obsolete pages.
> + *
> + * It's required when memslot is being deleted or VM is being
> + * destroyed, in these cases, we should ensure that KVM MMU does
> + * not use any resource of the being-deleted slot or all slots
> + * after calling the function.
> + */
> +static void kvm_mmu_zap_all_fast(struct kvm *kvm)
> +{
> +	write_lock(&kvm->mmu_lock);
> +	__kvm_mmu_zap_all_fast_front_half(kvm);
> +	write_unlock(&kvm->mmu_lock);
> +
> +	__kvm_mmu_zap_all_fast_back_half(kvm);
> +}
> +
>  int kvm_mmu_init_vm(struct kvm *kvm)
>  {
>  	int r, i;
> -- 
> 2.55.0.rc0.799.gd6f94ed593-goog
>