Re: [PATCH v2 4/7] KVM: selftests: Correctly initialize the VA space for TTBR0_EL1

Sean Christopherson
2022-12-09T20:45:15Z

On Fri, Dec 09, 2022, Oliver Upton wrote:
> An interesting feature of the Arm architecture is that the stage-1 MMU
> supports two distinct VA regions, controlled by TTBR{0,1}_EL1. As KVM
> selftests on arm64 only uses TTBR0_EL1, the VA space is constrained to
> [0, 2^(va_bits)). This is different from other architectures that
> allow for addressing low and high regions of the VA space from a single
> page table.
>
> KVM selftests' VA space allocator presumes the valid address range is
> split between low and high memory based the MSB, which of course is a
> poor match for arm64's TTBR0 region.
>
> Add a helper that correctly handles both addressing schemes with a
> comment describing each.
>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---

Thanks much! Looks awesome, especially the comment!

Reviewed-by: Sean Christopherson <seanjc@google.com>

> .../selftests/kvm/include/kvm_util_base.h | 1 +
> tools/testing/selftests/kvm/lib/kvm_util.c | 49 ++++++++++++++++---
> 2 files changed, 44 insertions(+), 6 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
> index 6cd86da698b3..b193863d754f 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util_base.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
> @@ -103,6 +103,7 @@ struct kvm_vm {
> struct sparsebit *vpages_mapped;
> bool has_irqchip;
> bool pgd_created;
> + bool has_split_va_space;
> vm_paddr_t ucall_mmio_addr;
> vm_paddr_t pgd;
> vm_vaddr_t gdt;
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index a256ec67aff6..53d15f32f220 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -186,6 +186,43 @@ const struct vm_guest_mode_params vm_guest_mode_params[] = {
> _Static_assert(sizeof(vm_guest_mode_params)/sizeof(struct vm_guest_mode_params) == NUM_VM_MODES,
> "Missing new mode params?");
>
> +/*
> + * Initializes vm->vpages_valid to match the canonical VA space of the
> + * architecture.
> + *
> + * Most architectures split the range addressed by a single page table into a
> + * low and high region based on the MSB of the VA. On architectures with this
> + * behavior the VA region spans [0, 2^(va_bits - 1)), [-(2^(va_bits - 1), -1].
> + *
> + * arm64 is a bit different from the rest of the crowd, as the low and high
> + * regions of the VA space are addressed by distinct paging structures
> + * (TTBR{0,1}_EL1).

Oooh, they're different CR3s in x86 terminology?