Re: [PATCH 2/2] KVM: arm64: Ensure TLBs and I-cache are private to each vCPU in pKVM

[Joey Gouly <joey.gouly@arm.com>]

Hi!

On Wed, Oct 14, 2026 at 01:55:10PM +0100, Fuad Tabba wrote:
> All vCPUs of a VM share its VMID, so KVM flushes a physical CPU's TLBs
> and I-cache for the VM whenever a different vCPU of that VM is loaded
> there (vcpu_prepare_mmu()). Commit fce886a602073 ("KVM: arm64: Plumb the
> pKVM MMU in KVM") moved pKVM guests, protected or not, onto the
> hypervisor's stage-2 and VMID, and the host stopped flushing for them.
> Nothing at EL2 flushes instead, so a vCPU can hit TLB and I-cache
> entries that another vCPU of the same VM left on that CPU.
> 
> Track the last vCPU loaded on each physical CPU in the hyp VM, and flush
> at EL2 in __pkvm_vcpu_load() when it changes.
> 
> Fixes: fce886a602073 ("KVM: arm64: Plumb the pKVM MMU in KVM")
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/pkvm.h |  3 +++
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c     |  8 ++++++++
>  arch/arm64/kvm/hyp/nvhe/pkvm.c         | 21 ++++++++++++++++++---
>  3 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/pkvm.h b/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
> index 9abfe6b7e942b..02c8fc7a10904 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
> @@ -54,6 +54,9 @@ struct pkvm_hyp_vm {
>  	unsigned short refcount;
>  	hyp_spinlock_t lock;
>  
> +	/* Per physical CPU, the vcpu_idx of the last vCPU of this VM loaded there. */
> +	int *last_vcpu_ran;

Trying to think of a better way to reword this, it's an array indexed by
pCPU, not per cpu in the normal linux way. Unlike `mmu->last_vcpu_ran`
which really is per_cpu. Maybe I'm just being nitpicky, and it's fine as
is!

> +
>  	/*
>  	 * The vCPU initialised RUNNABLE: claimed under vm_table_lock,
>  	 * released only if its own init fails.
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index d715aa942577e..f0b77236c871e 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -827,11 +827,19 @@ DEFINE_KVM_HOST_HCALL(void, __pkvm_vcpu_load,
>  	pkvm_handle_t, handle, unsigned int, vcpu_idx, u64, hcr_el2)
>  {
>  	struct pkvm_hyp_vcpu *hyp_vcpu;
> +	int *last_ran;
>  
>  	hyp_vcpu = pkvm_load_hyp_vcpu(handle, vcpu_idx);
>  	if (!hyp_vcpu)
>  		return;
>  
> +	/* TLBs and I-cache are private to each vCPU, see vcpu_prepare_mmu(). */
> +	last_ran = &pkvm_hyp_vcpu_to_hyp_vm(hyp_vcpu)->last_vcpu_ran[hyp_smp_processor_id()];
> +	if (*last_ran != vcpu_idx) {
> +		__kvm_flush_cpu_context(hyp_vcpu->vcpu.arch.hw_mmu);
> +		*last_ran = vcpu_idx;
> +	}
> +
>  	if (pkvm_hyp_vcpu_is_protected(hyp_vcpu)) {
>  		u64 dfr0 = read_sysreg(id_aa64dfr0_el1);
>  
> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> index ac020d0501d2e..f18fa762746c9 100644
> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> @@ -489,10 +489,11 @@ static void unpin_host_vcpus(struct pkvm_hyp_vcpu *hyp_vcpus[],
>  }
>  
>  static void init_pkvm_hyp_vm(struct kvm *host_kvm, struct pkvm_hyp_vm *hyp_vm,
> -			     unsigned int nr_vcpus, pkvm_handle_t handle)
> +			     int *last_vcpu_ran, unsigned int nr_vcpus, pkvm_handle_t handle)
>  {
>  	struct kvm_s2_mmu *mmu = &hyp_vm->kvm.arch.mmu;
>  	int idx = vm_handle_to_idx(handle);
> +	unsigned long cpu;
>  
>  	hyp_vm->kvm.arch.pkvm.handle = handle;
>  
> @@ -512,6 +513,10 @@ static void init_pkvm_hyp_vm(struct kvm *host_kvm, struct pkvm_hyp_vm *hyp_vm,
>  	mmu->vtcr = host_mmu.arch.mmu.vtcr;
>  	mmu->arch = &hyp_vm->kvm.arch;
>  	mmu->pgt = &hyp_vm->pgt;
> +
> +	for (cpu = 0; cpu < hyp_nr_cpus; cpu++)
> +		last_vcpu_ran[cpu] = -1;
> +	hyp_vm->last_vcpu_ran = last_vcpu_ran;
>  }
>  
>  static int pkvm_vcpu_init_sve(struct pkvm_hyp_vcpu *hyp_vcpu, struct kvm_vcpu *host_vcpu)
> @@ -887,6 +892,7 @@ int __pkvm_init_vm(struct kvm *host_kvm, void __kern *pgd_hva)
>  	size_t vm_size, pgd_size;
>  	unsigned int nr_vcpus;
>  	pkvm_handle_t handle;
> +	int *last_vcpu_ran;
>  	void *pgd = NULL;
>  	int ret;
>  
> @@ -915,13 +921,19 @@ int __pkvm_init_vm(struct kvm *host_kvm, void __kern *pgd_hva)
>  		goto err_unpin_kvm;
>  	}
>  
> +	last_vcpu_ran = hyp_alloc(array_size(hyp_nr_cpus, sizeof(*last_vcpu_ran)));
> +	if (!last_vcpu_ran) {
> +		ret = hyp_alloc_errno();
> +		goto err_free_hyp_vm;
> +	}
> +
>  	ret = -ENOMEM;
>  
>  	pgd = map_donated_memory_noclear(pgd_hva, pgd_size);
>  	if (!pgd)
> -		goto err_free_hyp_vm;
> +		goto err_free_last_ran;
>  
> -	init_pkvm_hyp_vm(host_kvm, hyp_vm, nr_vcpus, handle);
> +	init_pkvm_hyp_vm(host_kvm, hyp_vm, last_vcpu_ran, nr_vcpus, handle);
>  
>  	ret = kvm_guest_prepare_stage2(hyp_vm, pgd);
>  	if (ret)
> @@ -938,6 +950,8 @@ int __pkvm_init_vm(struct kvm *host_kvm, void __kern *pgd_hva)
>  	kvm_guest_destroy_stage2(hyp_vm);
>  err_remove_mappings:
>  	unmap_donated_memory(pgd, pgd_size);
> +err_free_last_ran:
> +	hyp_free(last_vcpu_ran);
>  err_free_hyp_vm:
>  	hyp_free(hyp_vm);
>  err_unpin_kvm:
> @@ -1112,6 +1126,7 @@ int __pkvm_finalize_teardown_vm(pkvm_handle_t handle)
>  		hyp_free(hyp_vcpu);
>  	}
>  
> +	hyp_free(hyp_vm->last_vcpu_ran);
>  	hyp_free(hyp_vm);
>  	hyp_unpin_shared_mem(host_kvm, host_kvm + 1);
>  	return 0;

For the series:
Reviewed-by: Joey Gouly <joey.gouly@arm.com>

Thanks,
Joey