Re: [PATCH v3 55/60] arm64: kvm: avoid CONFIG_PGTABLE_LEVELS for runtime levels

Ryan Roberts
2023-04-18T14:29:58Z

On 07/03/2023 14:05, Ard Biesheuvel wrote:
> get_user_mapping_size() uses vabits_actual and CONFIG_PGTABLE_LEVELS to
> provide the starting point for a table walk. This is fine for LVA, as
> the number of translation levels is the same regardless of whether LVA
> is enabled. However, with LPA2, this will no longer be the case, so
> let's derive the number of levels from the number of VA bits directly.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/arm64/kvm/mmu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index d64be7b5f6692e8b..4e7c0f9a9c286c09 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -663,7 +663,7 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
> .pgd = (kvm_pteref_t)kvm->mm->pgd,
> .ia_bits = vabits_actual,
> .start_level = (KVM_PGTABLE_MAX_LEVELS -
> - CONFIG_PGTABLE_LEVELS),
> + ARM64_HW_PGTABLE_LEVELS(pgt.ia_bits)),
> .mm_ops = &kvm_user_mm_ops,
> };
> kvm_pte_t pte = 0; /* Keep GCC quiet... */

You have the problem here that the KVM library (which isn't LPA2 aware) is
walking a kernel page table, which may now be in LPA2 format. I think this works
out ok as long as there are no physical addresses above 48 bits in the page
table. But otherwise, I doubt it works out very well...