[PATCH v2 13/13] KVM: selftests: Add nested pre-fault test for arm64

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Add an arm64 nested-virtualisation selftest for KVM_PRE_FAULT_MEMORY.

The guest enters vEL2, sets up its EL2 translation configuration into the
real EL1 registers then ERETs to vEL1 and exits to userspace so its vCPU
last-run context is nested backed by a shadow stage 2 MMU.

The test then asserts that pre-faulting ignores that and instead targets
the canonical stage 2 page tables only.

Userspace cannot observe a mistargeted pre-fault, so simply assert that the
pre-faulting works correctly when the last-run context was nested.

Based on a patch by Jack Thomson with gratitude. The test here is
simplified and focused on what can be asserted.

Link: https://lore.kernel.org/kvmarm/20260612162354.73378-6-jackabt.amazon@gmail.com/
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 tools/testing/selftests/kvm/Makefile.kvm           |   1 +
 .../selftests/kvm/arm64/nv_pre_fault_memory_test.c | 158 +++++++++++++++++++++
 2 files changed, 159 insertions(+)

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 66a9a4e62888..1e0edd0f99f7 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -173,6 +173,7 @@ TEST_GEN_PROGS_arm64 += arm64/debug-exceptions
 TEST_GEN_PROGS_arm64 += arm64/hello_el2
 TEST_GEN_PROGS_arm64 += arm64/host_sve
 TEST_GEN_PROGS_arm64 += arm64/hypercalls
+TEST_GEN_PROGS_arm64 += arm64/nv_pre_fault_memory_test
 TEST_GEN_PROGS_arm64 += arm64/external_aborts
 TEST_GEN_PROGS_arm64 += arm64/mmio_sign_ext
 TEST_GEN_PROGS_arm64 += arm64/page_fault_test
diff --git a/tools/testing/selftests/kvm/arm64/nv_pre_fault_memory_test.c b/tools/testing/selftests/kvm/arm64/nv_pre_fault_memory_test.c
new file mode 100644
index 000000000000..09c1db303856
--- /dev/null
+++ b/tools/testing/selftests/kvm/arm64/nv_pre_fault_memory_test.c
@@ -0,0 +1,158 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * nv_pre_fault_memory_test - Test KVM_PRE_FAULT_MEMORY on a vCPU whose
+ * last-run context is nested.
+ *
+ * The guest enters vEL2, sets up its EL2 translation configuration into the
+ * real EL1 registers then ERETs to vEL1 and exits to userspace so its vCPU
+ * last-run context is nested backed by a shadow stage 2 MMU.
+ *
+ * Assert that pre-faulting ignores that and targets the canonical stage-2
+ * page tables only.
+ */
+#include "kvm_util.h"
+#include "processor.h"
+#include "test_util.h"
+#include "ucall.h"
+
+#include <asm/sysreg.h>
+#include <linux/sizes.h>
+
+#define TEST_MEM_SLOT		10
+#define TEST_MEM_SIZE		SZ_2M
+#define TEST_MEM_GPA		SZ_1G
+
+static void guest_el1_code(void)
+{
+	u64 offset;
+
+	GUEST_ASSERT_EQ(get_current_el(), 1);
+
+	/* Exit to userspace with the vEL1 (nested) context live. */
+	GUEST_SYNC(1);
+
+	/*
+	 * Touch the prefaulted range. vstage-2 is disabled, so the shadow
+	 * stage-2 is a 1:1 view of the canonical IPA space.
+	 */
+	for (offset = 0; offset < TEST_MEM_SIZE; offset += SZ_4K)
+		READ_ONCE(*(u64 *)(TEST_MEM_GPA + offset));
+
+	GUEST_DONE();
+}
+
+static void guest_code(void)
+{
+	u64 sp;
+
+	GUEST_ASSERT_EQ(get_current_el(), 2);
+
+	/*
+	 * Mirror the EL2 translation regime into the real EL1 registers so
+	 * that vEL1 runs on the test's stage-1 page tables. With E2H=1, the
+	 * _EL1 accessors read the EL2 registers, and the _EL12 accessors
+	 * write the real EL1 registers.
+	 */
+	write_sysreg_s(read_sysreg(sctlr_el1), SYS_SCTLR_EL12);
+	write_sysreg_s(read_sysreg(tcr_el1), SYS_TCR_EL12);
+	write_sysreg_s(read_sysreg(ttbr0_el1), SYS_TTBR0_EL12);
+	write_sysreg_s(read_sysreg(mair_el1), SYS_MAIR_EL12);
+	write_sysreg_s(read_sysreg(cpacr_el1), SYS_CPACR_EL12);
+
+	/* Run vEL1 on the same stack. */
+	asm volatile("mov %0, sp" : "=r"(sp));
+	write_sysreg(sp, sp_el1);
+
+	/*
+	 * Drop TGE so that vEL1 is a nested context rather than host EL0.
+	 * KVM backs it with a shadow stage-2 MMU even though vstage-2 is
+	 * disabled (HCR_EL2.VM=0).
+	 */
+	write_sysreg(read_sysreg(hcr_el2) & ~HCR_EL2_TGE, hcr_el2);
+	isb();
+
+	write_sysreg(PSR_MODE_EL1h | PSR_F_BIT | PSR_I_BIT | PSR_A_BIT |
+		     PSR_D_BIT, spsr_el2);
+	write_sysreg((u64)guest_el1_code, elr_el2);
+	asm volatile("eret");
+
+	GUEST_ASSERT(false);
+}
+
+static void pre_fault(struct kvm_vcpu *vcpu, u64 gpa, u64 size)
+{
+	struct kvm_pre_fault_memory range = {
+		.gpa = gpa,
+		.size = size,
+	};
+	int ret;
+
+	do {
+		ret = __vcpu_ioctl(vcpu, KVM_PRE_FAULT_MEMORY, &range);
+	} while ((!ret && range.size) ||
+		 (ret < 0 && (errno == EINTR || errno == EAGAIN)));
+
+	TEST_ASSERT(!ret, "KVM_PRE_FAULT_MEMORY failed, ret: %d errno: %d",
+		    ret, errno);
+	TEST_ASSERT_EQ(range.size, 0);
+}
+
+int main(void)
+{
+	struct kvm_vcpu_init init;
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	struct ucall uc;
+	u64 npages;
+
+	TEST_REQUIRE(test_supports_el2());
+	TEST_REQUIRE(kvm_check_cap(KVM_CAP_PRE_FAULT_MEMORY));
+
+	vm = vm_create(1);
+
+	kvm_get_default_vcpu_target(vm, &init);
+	init.features[0] |= BIT(KVM_ARM_VCPU_HAS_EL2);
+	vcpu = aarch64_vcpu_add(vm, 0, &init, guest_code);
+	kvm_arch_vm_finalize_vcpus(vm);
+
+	npages = TEST_MEM_SIZE / vm->page_size;
+	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS, TEST_MEM_GPA,
+				    TEST_MEM_SLOT, npages, 0);
+	virt_map(vm, TEST_MEM_GPA, TEST_MEM_GPA, npages);
+
+	/* Run the guest until it has ERET'd from vEL2 to vEL1. */
+	vcpu_run(vcpu);
+	switch (get_ucall(vcpu, &uc)) {
+	case UCALL_SYNC:
+		TEST_ASSERT_EQ(uc.args[1], 1);
+		break;
+	case UCALL_ABORT:
+		REPORT_GUEST_ASSERT(uc);
+		break;
+	default:
+		TEST_FAIL("Unhandled ucall: %ld", uc.cmd);
+	}
+
+	/*
+	 * The vCPU's last-run context is vEL1, so its hw_mmu is a shadow
+	 * stage-2 MMU.
+	 *
+	 * Pre-faulting must ignore that and populate the canonical stage-2.
+	 */
+	pre_fault(vcpu, TEST_MEM_GPA, TEST_MEM_SIZE);
+
+	/* Resume at vEL1 and touch the prefaulted range. */
+	vcpu_run(vcpu);
+	switch (get_ucall(vcpu, &uc)) {
+	case UCALL_DONE:
+		break;
+	case UCALL_ABORT:
+		REPORT_GUEST_ASSERT(uc);
+		break;
+	default:
+		TEST_FAIL("Unhandled ucall: %ld", uc.cmd);
+	}
+
+	kvm_vm_free(vm);
+	return 0;
+}

-- 
2.55.0