Re: [PATCH v3 09/12] KVM: x86/mmu: Use split "zap all fast" helpers when invalidating memslot

[Michael Roth]

On Tue, Jun 30, 2026 at 03:26:04PM -0700, Sean Christopherson wrote:
> Manually invoke the front half and back half of the "zap all fast" flow
> when invalidating a memslot so that mmu_lock is acquired at function scope
> in kvm_arch_flush_shadow_memslot().   This will allow putting more code
> inside the critical section without having to take mmu_lock twice in quick
> succession.
> 
> Opportunistically open code checking whether or not to do the fast zap, to
> discourage removing the local "zap_all" in a future cleanup, i.e. to ensure
> the SLOT_ZAP_ALL quirk is queried exactly once.  Processing the front half

Enforcement through finger punishment? That sounds surprisingly effective :)

> but not the back half of the fast zap (if SLOT_ZAP_ALL were disabled
> concurrently) would result in KVM unnecessarily keeping invalid TDP MMU
> roots until the VM is destroyed.
> 
> 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 | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index a5c2a560a88a..3eb1f86593b1 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -7573,12 +7573,6 @@ static void kvm_mmu_zap_memslot_pages_and_flush(struct kvm *kvm,
>  	kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, flush);
>  }
>  
> -static inline bool kvm_memslot_flush_zap_all(struct kvm *kvm)
> -{
> -	return kvm->arch.vm_type == KVM_X86_DEFAULT_VM &&
> -	       kvm_check_has_quirk(kvm, KVM_X86_QUIRK_SLOT_ZAP_ALL);
> -}
> -
>  void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
>  				   struct kvm_memory_slot *slot)
>  {
> @@ -7589,16 +7583,23 @@ void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
>  		.may_block = true,
>  		.attr_filter = KVM_FILTER_PRIVATE | KVM_FILTER_SHARED,
>  	};
> +	bool zap_all = kvm->arch.vm_type == KVM_X86_DEFAULT_VM &&
> +		       kvm_check_has_quirk(kvm, KVM_X86_QUIRK_SLOT_ZAP_ALL);
>  	bool flush;
>  
> -	if (kvm_memslot_flush_zap_all(kvm)) {
> -		kvm_mmu_zap_all_fast(kvm);
> +	write_lock(&kvm->mmu_lock);
> +
> +	if (zap_all) {
> +		__kvm_mmu_zap_all_fast_front_half(kvm);
>  	} else {
> -		write_lock(&kvm->mmu_lock);
>  		flush = kvm_unmap_gfn_range(kvm, &range);
>  		kvm_mmu_zap_memslot_pages_and_flush(kvm, slot, flush);
> -		write_unlock(&kvm->mmu_lock);
>  	}
> +
> +	write_unlock(&kvm->mmu_lock);
> +
> +	if (zap_all)
> +		__kvm_mmu_zap_all_fast_back_half(kvm);
>  }
>  
>  void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen)
> -- 
> 2.55.0.rc0.799.gd6f94ed593-goog
>