Re: [PATCH v2] KVM: arm64: Ensure TLBs and I-cache are private to each vCPU in pKVM

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

On Tue, Oct 20, 2026 at 08:43:11AM +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. pKVM guests, protected or not, run on the hypervisor's stage-2
> and VMID, and the host doesn't flush 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 at EL2 the last vCPU of each VM loaded on each CPU, and flush
> when it changes. EL2 can't allocate per-CPU memory for each VM, as the
> host does, so use one per-CPU array indexed by the VM's table entry.
> Reset the entry's slot on every CPU when a VM reserves it, so that a new
> VM starts with no last vCPU, whether the entry is unused or was left by
> a VM that was torn down.
> 
> Fixes: fce886a602073 ("KVM: arm64: Plumb the pKVM MMU in KVM")
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>

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

> ---
> Changes since v1 [1]:
>   - Dropped patch 1, which added a CPU number at EL2. The last vCPU is
>     now tracked in a per-CPU array indexed by VM table entry. (Marc)
>   - Moved the check from __pkvm_vcpu_load() to pkvm_load_hyp_vcpu().
>   - The array is unsigned int, initialised to UINT_MAX. (Vincent)
>   - Dropped Joey's Reviewed-by, since the patch changed.
> 
> Based on kvmarm/next (dd0bc00e3e920).
> 
> [1] https://lore.kernel.org/all/20261014125510.2928306-1-fuad.tabba@linux.dev/
> 
>  arch/arm64/kvm/hyp/nvhe/pkvm.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> index ac020d0501d2e..1fc59229c8e86 100644
> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> @@ -38,6 +38,9 @@ unsigned int kvm_host_sve_max_vl;
>   */
>  static DEFINE_PER_CPU(struct pkvm_hyp_vcpu *, loaded_hyp_vcpu);
>  
> +/* Per VM table entry, the vcpu_idx of the last vCPU of that VM loaded on this CPU. */
> +static DEFINE_PER_CPU(unsigned int, last_vcpu_ran[KVM_MAX_PVMS]);
> +
>  static void pkvm_vcpu_reset_hcr(struct kvm_vcpu *vcpu)
>  {
>  	vcpu->arch.hcr_el2 = HCR_GUEST_FLAGS;
> @@ -261,6 +264,7 @@ struct pkvm_hyp_vcpu *pkvm_load_hyp_vcpu(pkvm_handle_t handle,
>  {
>  	struct pkvm_hyp_vcpu *hyp_vcpu = NULL;
>  	struct pkvm_hyp_vm *hyp_vm;
> +	unsigned int *last_ran;
>  
>  	/* Cannot load a new vcpu without putting the old one first. */
>  	if (__this_cpu_read(loaded_hyp_vcpu))
> @@ -290,8 +294,18 @@ struct pkvm_hyp_vcpu *pkvm_load_hyp_vcpu(pkvm_handle_t handle,
>  unlock:
>  	hyp_spin_unlock(&vm_table_lock);
>  
> -	if (hyp_vcpu)
> -		__this_cpu_write(loaded_hyp_vcpu, hyp_vcpu);
> +	if (!hyp_vcpu)
> +		return NULL;
> +
> +	__this_cpu_write(loaded_hyp_vcpu, hyp_vcpu);
> +
> +	/* TLBs and I-cache are private to each vCPU, see vcpu_prepare_mmu(). */
> +	last_ran = this_cpu_ptr(&last_vcpu_ran[vm_handle_to_idx(handle)]);
> +	if (*last_ran != vcpu_idx) {
> +		__kvm_flush_cpu_context(hyp_vcpu->vcpu.arch.hw_mmu);
> +		*last_ran = vcpu_idx;
> +	}
> +
>  	return hyp_vcpu;
>  }
>  
> @@ -652,6 +666,7 @@ static int find_free_vm_table_entry(void)
>   */
>  static int allocate_vm_table_entry(void)
>  {
> +	unsigned int cpu;
>  	int idx;
>  
>  	hyp_assert_lock_held(&vm_table_lock);
> @@ -670,6 +685,9 @@ static int allocate_vm_table_entry(void)
>  
>  	vm_table[idx] = RESERVED_ENTRY;
>  
> +	for (cpu = 0; cpu < hyp_nr_cpus; cpu++)
> +		per_cpu(last_vcpu_ran[idx], cpu) = UINT_MAX;
> +
>  	return idx;
>  }
>  
> 
> base-commit: dd0bc00e3e9201fd49b62ff9cc07890c89003afe
> -- 
> 2.47.3
>