linux-kernel - Re: [PATCH v2 4/7] KVM: selftests: Correctly initialize the VA space for TTBR0

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Date:   Mon, 12 Dec 2022 11:34:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Oliver Upton <oliver.upton@...ux.dev>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Shuah Khan <shuah@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        Ricardo Koller <ricarkol@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/7] KVM: selftests: Correctly initialize the VA space
 for TTBR0_EL1

On 12/9/22 02:53, Oliver Upton wrote:
> @@ -268,17 +305,17 @@ struct kvm_vm *____vm_create(enum vm_guest_mode mode)
>   #ifdef __aarch64__
>   	if (vm->pa_bits != 40)
>   		vm->type = KVM_VM_TYPE_ARM_IPA_SIZE(vm->pa_bits);
> +
> +	/* selftests use TTBR0 only, meaning there is a single VA region. */
> +	vm->has_split_va_space = false;
> +#else
> +	vm->has_split_va_space = true;
>   #endif
>   

Even though there happens to be already a suitable #ifdef, I don't
really like them and don't think there should be a new bool unless
something else uses it.

However, the new comment is very useful, so I added it to kvm_util.c as
follows:

/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index 759a45540108..56d5ea949cbb 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -186,6 +186,15 @@ 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.
+ *
+ * The default implementation is valid for architectures which 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].
+ */
  __weak void vm_vaddr_populate_bitmap(struct kvm_vm *vm)
  {
  	sparsebit_set_num(vm->vpages_valid,