Re: [PATCH 1/6] KVM: arm64: Remove VM-wide VNCR mapping counter

On Sat, Aug 01, 2026 at 01:48:13PM +0800, Marc Zyngier wrote:
> The global VNCR mapping counter is used to decide whether an L1
> provided VNCR page is mapped in L0 on any CPU at the point of
> dealing with a TLB invalidation. It is incremented when a mapping
> is made in the fixmap, and decremented when unmapped.
>
> As it turns out, this tracking has several flaws:
>
> - we are trying to invalidate TLBs, and the mapping is only an
>   opportunistic consequence of the TLB. Checking this counter to
>   decide whether a TLB needs to be invalidated may result in missed
>   invalidations.
>
> - an L1 vcpu invalidating its own TLB (a very likely case) will not
>   succeed in invalidating the VNCR pseudo TLB because that page is
>   not mapped in L0 at this stage.

Agree, the easiest way to trigger the issue is run single
vcpu L1 and run single vcpu L2. I think it's more harder to
hit the issue if the L1 has more vcpus and run L2 w/ more
than 1 L2 vcpus.

Reviewed-by: Yuan Yao <[email protected]>

>
> Given that this tracking fails at delivering the minimum guarantees
> that are required and is only a performance optimisation, remove it
> completely.
>
> Fixes: 4ffa72ad8f37e ("KVM: arm64: nv: Add S1 TLB invalidation primitive for VNCR_EL2")
> Signed-off-by: Marc Zyngier <[email protected]>
> Cc: [email protected]
> ---
>  arch/arm64/include/asm/kvm_host.h | 3 ---
>  arch/arm64/kvm/hyp/vhe/switch.c   | 3 +--
>  arch/arm64/kvm/nested.c           | 3 ---
>  3 files changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index bae2c4f92ef5c..ac16f96c878d6 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -411,9 +411,6 @@ struct kvm_arch {
>  	/* Masks for VNCR-backed and general EL2 sysregs */
>  	struct kvm_sysreg_masks	*sysreg_masks;
>
> -	/* Count the number of VNCR_EL2 currently mapped */
> -	atomic_t vncr_map_count;
> -
>  	/*
>  	 * For an untrusted host VM, 'pkvm.handle' is used to lookup
>  	 * the associated pKVM instance in the hypervisor.
> diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> index bbe9cebd3d9d5..c09b1d411c584 100644
> --- a/arch/arm64/kvm/hyp/vhe/switch.c
> +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> @@ -427,8 +427,7 @@ static bool kvm_hyp_handle_tlbi_el2(struct kvm_vcpu *vcpu, u64 *exit_code)
>  	 * If we have to check for any VNCR mapping being invalidated,
>  	 * go back to the slow path for further processing.
>  	 */
> -	if (vcpu_el2_e2h_is_set(vcpu) && vcpu_el2_tge_is_set(vcpu) &&
> -	    atomic_read(&vcpu->kvm->arch.vncr_map_count))
> +	if (vcpu_el2_e2h_is_set(vcpu) && vcpu_el2_tge_is_set(vcpu))
>  		return false;
>
>  	__kvm_skip_instr(vcpu);
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index dfb96edbdc43c..f3c75954cf36c 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -48,7 +48,6 @@ void kvm_init_nested(struct kvm *kvm)
>  {
>  	kvm->arch.nested_mmus = NULL;
>  	kvm->arch.nested_mmus_size = 0;
> -	atomic_set(&kvm->arch.vncr_map_count, 0);
>  }
>
>  static int init_nested_s2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu)
> @@ -890,7 +889,6 @@ static void this_cpu_reset_vncr_fixmap(struct kvm_vcpu *vcpu)
>  	clear_fixmap(vncr_fixmap(vcpu->arch.vncr_tlb->cpu));
>  	vcpu->arch.vncr_tlb->cpu = -1;
>  	host_data_clear_flag(L1_VNCR_MAPPED);
> -	atomic_dec(&vcpu->kvm->arch.vncr_map_count);
>  }
>
>  void kvm_vcpu_put_hw_mmu(struct kvm_vcpu *vcpu)
> @@ -1592,7 +1590,6 @@ static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)
>  	if (pgprot_val(prot) != pgprot_val(PAGE_NONE)) {
>  		__set_fixmap(vncr_fixmap(vt->cpu), vt->hpa, prot);
>  		host_data_set_flag(L1_VNCR_MAPPED);
> -		atomic_inc(&vcpu->kvm->arch.vncr_map_count);
>  	}
>  }
>
> --
> 2.47.3
>