linux-kernel - Re: [PATCH 3/4] KVM: arm64: selftests: Align VA space allocator with TTBR0

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Message-ID: <Y5EtP5z6rxSK1VUe@google.com>
Date:   Thu, 8 Dec 2022 00:18:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        Ricardo Koller <ricarkol@...gle.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: arm64: selftests: Align VA space allocator with
 TTBR0

On Wed, Dec 07, 2022, Oliver Upton wrote:
> diff --git a/tools/testing/selftests/kvm/lib/aarch64/processor.c b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> index 316de70db91d..5972a23b2765 100644
> --- a/tools/testing/selftests/kvm/lib/aarch64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> @@ -541,3 +541,13 @@ void kvm_selftest_arch_init(void)
>  	 */
>  	guest_modes_append_default();
>  }
> +
> +void vm_vaddr_populate_bitmap(struct kvm_vm *vm)

Add "arch" so that it's obvious this can be overidden?  The "__weak" conveys that
for the implementation, but not for the call site.  E.g. vm_arch_vaddr_populate_bitmap().

Actually, IIUC, the issue is that the high half isn't mapped (probably the wrong
terminology).  I.e. the calculation for the low half stays the same, and the high
half just goes away.

> +{
> +	/*
> +	 * arm64 selftests use only TTBR0_EL1, meaning that the valid VA space
> +	 * is [0, 2^(64 - TCR_EL1.T0SZ)).
> +	 */
> +	sparsebit_set_num(vm->vpages_valid, 0,
> +			  (1ULL << vm->va_bits) >> vm->page_shift);
> +}
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index e9607eb089be..c88c3ace16d2 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?");
>  
> +__weak void vm_vaddr_populate_bitmap(struct kvm_vm *vm)
> +{
> +	sparsebit_set_num(vm->vpages_valid,
> +		0, (1ULL << (vm->va_bits - 1)) >> vm->page_shift);
> +	sparsebit_set_num(vm->vpages_valid,
> +		(~((1ULL << (vm->va_bits - 1)) - 1)) >> vm->page_shift,
> +		(1ULL << (vm->va_bits - 1)) >> vm->page_shift);

Any objection to fixing up the formatting?  Actually, we can do more than just
fix the indentation, e.g. the number of bits is identical, and documenting that
this does a high/low split would be helpful.

Together, what about?  The #ifdef is a bit gross, especially around "hi_start",
but it's less duplicate code.  And IMO, having things bundled in the same place
makes it a lot easier for newbies (to arm64 or kernel coding in general) to
understand what's going on and why arm64 is different.

---
 tools/testing/selftests/kvm/lib/kvm_util.c | 23 +++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index e9607eb089be..d6f2c17e3d40 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -186,6 +186,23 @@ 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?");
 
+static void vm_vaddr_populate_bitmap(struct kvm_vm *vm)
+{
+	/*
+	 * All architectures supports splitting the virtual address space into
+	 * a high and a low half.  Populate both halves, except for arm64 which
+	 * currently uses only TTBR0_EL1 (arbitrary selftests "logic"), i.e.
+	 * only has a valid low half.
+	 */
+	sparsebit_num_t nr_va_bits = (1ULL << (vm->va_bits - 1)) >> vm->page_shift;
+#ifndef __aarch64__
+	sparsebit_num_t hi_start = (~((1ULL << (vm->va_bits - 1)) - 1)) >> vm->page_shift
+
+	sparsebit_set_num(vm->vpages_valid, hi_start, nr_bits);
+#endif
+	sparsebit_set_num(vm->vpages_valid, 0, nr_va_bits);
+}
+
 struct kvm_vm *____vm_create(enum vm_guest_mode mode)
 {
 	struct kvm_vm *vm;
@@ -274,11 +291,7 @@ struct kvm_vm *____vm_create(enum vm_guest_mode mode)
 
 	/* Limit to VA-bit canonical virtual addresses. */
 	vm->vpages_valid = sparsebit_alloc();
-	sparsebit_set_num(vm->vpages_valid,
-		0, (1ULL << (vm->va_bits - 1)) >> vm->page_shift);
-	sparsebit_set_num(vm->vpages_valid,
-		(~((1ULL << (vm->va_bits - 1)) - 1)) >> vm->page_shift,
-		(1ULL << (vm->va_bits - 1)) >> vm->page_shift);
+	vm_vaddr_populate_bitmap(vm);
 
 	/* Limit physical addresses to PA-bits. */
 	vm->max_gfn = vm_compute_max_gfn(vm);

base-commit: 35aecc3289eebf193fd70a067ea448ae2f0bb9b9
--