Re: [PATCH 8/8] KVM: selftests: Add nested pre-fault test for arm64

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

+cc Wei-Lin

On Fri, Sep 11, 2026 at 03:30:41PM +0900, Itaru Kitayama wrote:
> Hi Lorenzo,
>
> On Tue, Aug 25, 2026 at 05:00:42PM +0100, Lorenzo Stoakes (ARM) wrote:
> > From: Jack Thomson <jackabt@amazon.com>
> >
> > Add an arm64 nested-virt selftest for KVM_PRE_FAULT_MEMORY. The guest
> > enters vEL1 and exits to userspace with a nested/shadow stage-2 MMU as
> > the vCPU's last-run context.
> >
> > Before prefaulting, userspace enables HCR_EL2.VM and points VTTBR_EL2 at
> > an empty nested stage-2 root. A prefault implementation that incorrectly
> > treats the userspace GPA as an L2 IPA will fail the ioctl; the correct
> > path targets the canonical stage-2 and succeeds.
> >
> > Restore the original nested state before resuming the guest, then touch
> > the prefaulted range to check that vEL1 still runs correctly.
> >
> > Signed-off-by: Jack Thomson <jackabt@amazon.com>
> > [ljs: partial progress, >4 KiB pgsize, commit msg, comment fixups]
> > 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 | 206 +++++++++++++++++++++
> >  2 files changed, 207 insertions(+)
>
> I wonder whether this selftest can use functions Wei-Lin proposed
> some time ago [1]: 20260325003620.2214766-1-weilin.chang@arm.com
> or would you prefer this test as propose, since it is clear as
> to what needs to be done to enter L2.

I think if these aren't merged yet then that could be a follow up?

>
> Thanks,
> Itaru.
>
> >
> > 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..b3833aea2a34
> > --- /dev/null
> > +++ b/tools/testing/selftests/kvm/arm64/nv_pre_fault_memory_test.c
> > @@ -0,0 +1,206 @@
> > +// 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 starts at vEL2, mirrors its EL2 translation regime into the
> > + * real EL1 registers, drops HCR_EL2.TGE and ERETs to vEL1, then exits to
> > + * userspace from vEL1 so that the vCPU's last-run context selects a
> > + * shadow stage-2 MMU. Userspace then enables an empty nested stage-2
> > + * before prefaulting. Prefaulting must target the canonical stage-2,
> > + * regardless of the vCPU's nested state.
> > + */
> > +#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 NESTED_S2_ROOT_SLOT	11
> > +#define TEST_MEM_SIZE		SZ_2M
> > +#define TEST_MEM_GPA		SZ_1G
> > +#define NESTED_S2_ROOT_GPA	(TEST_MEM_GPA + TEST_MEM_SIZE)
> > +
> > +struct nested_s2_state {
> > +	u64 hcr_el2;
> > +	u64 vttbr_el2;
> > +};
> > +
> > +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);
> > +}
> > +
> > +static struct nested_s2_state enable_empty_nested_s2(struct kvm_vcpu *vcpu)
> > +{
> > +	struct nested_s2_state state = {
> > +		.hcr_el2 = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(SYS_HCR_EL2)),
> > +		.vttbr_el2 = vcpu_get_reg(vcpu,
> > +					   KVM_ARM64_SYS_REG(SYS_VTTBR_EL2)),
> > +	};
> > +
> > +	TEST_ASSERT(!(state.hcr_el2 & HCR_EL2_TGE),
> > +		    "vCPU should be in nested/vEL1 context");
> > +
> > +	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_VTTBR_EL2),
> > +		     NESTED_S2_ROOT_GPA);
> > +	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_HCR_EL2),
> > +		     state.hcr_el2 | HCR_EL2_VM);
> > +
> > +	return state;
> > +}
> > +
> > +static void restore_nested_s2(struct kvm_vcpu *vcpu,
> > +			      struct nested_s2_state *state)
> > +{
> > +	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_HCR_EL2), state->hcr_el2);
> > +	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_VTTBR_EL2),
> > +		     state->vttbr_el2);
> > +}
> > +
> > +int main(void)
> > +{
> > +	struct nested_s2_state s2;
> > +	struct kvm_vcpu_init init;
> > +	struct kvm_vcpu *vcpu;
> > +	struct kvm_vm *vm;
> > +	struct ucall uc;
> > +	u64 npages;
> > +
> > +	TEST_REQUIRE(kvm_check_cap(KVM_CAP_ARM_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);
> > +
> > +	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
> > +				    NESTED_S2_ROOT_GPA, NESTED_S2_ROOT_SLOT,
> > +				    vm_adjust_num_guest_pages(vm->mode, 1), 0);
> > +
> > +	/* 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, backed by a shadow stage-2
> > +	 * MMU. Enable nested stage-2 with an empty root so that the ioctl
> > +	 * fails if it tries to interpret the userspace GPA as an L2 IPA.
> > +	 *
> > +	 * Prefault in two halves so that the second ioctl exercises a
> > +	 * repeated shadow-MMU attach and canonical stage-2 swap.
> > +	 *
> > +	 * (Note that an implementation that wrongly populates shadow
> > +	 * stage-2 page tables would not be caught as userland can't
> > +	 * inspect these.)
> > +	 */
> > +	s2 = enable_empty_nested_s2(vcpu);
> > +	pre_fault(vcpu, TEST_MEM_GPA, TEST_MEM_SIZE / 2);
> > +	pre_fault(vcpu, TEST_MEM_GPA + TEST_MEM_SIZE / 2, TEST_MEM_SIZE / 2);
> > +	restore_nested_s2(vcpu, &s2);
> > +
> > +	/* 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
> >

--
Cheers, Lorenzo