Re: [PATCH 1/3] KVM: arm64: selftests: Add library functions for NV

On Wed, Mar 25, 2026 at 09:03:47AM +0000, Marc Zyngier wrote:

On Wed, 25 Mar 2026 00:36:18 +0000,
Wei-Lin Chang [off-list ref] wrote:

quoted

The API is designed for userspace to first call prepare_{l2_stack,
hyp_state, eret_destination, nested_sync_handler}, with a function
supplied to prepare_eret_destination() to be run in L2. Then run_l2()
can be called in L1 to run the given function in L2.

Signed-off-by: Wei-Lin Chang <redacted>
---
 tools/testing/selftests/kvm/Makefile.kvm      |  2 +
 .../selftests/kvm/include/arm64/nested.h      | 18 ++++++
 .../testing/selftests/kvm/lib/arm64/nested.c  | 61 +++++++++++++++++++
 .../selftests/kvm/lib/arm64/nested_asm.S      | 35 +++++++++++
 4 files changed, 116 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/include/arm64/nested.h
 create mode 100644 tools/testing/selftests/kvm/lib/arm64/nested.c
 create mode 100644 tools/testing/selftests/kvm/lib/arm64/nested_asm.S

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 98da9fa4b8b7..5e681e8e0cd7 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm

@@ -34,6 +34,8 @@ LIBKVM_arm64 += lib/arm64/gic.c
 LIBKVM_arm64 += lib/arm64/gic_v3.c
 LIBKVM_arm64 += lib/arm64/gic_v3_its.c
 LIBKVM_arm64 += lib/arm64/handlers.S
+LIBKVM_arm64 += lib/arm64/nested.c
+LIBKVM_arm64 += lib/arm64/nested_asm.S
 LIBKVM_arm64 += lib/arm64/processor.c
 LIBKVM_arm64 += lib/arm64/spinlock.c
 LIBKVM_arm64 += lib/arm64/ucall.c

diff --git a/tools/testing/selftests/kvm/include/arm64/nested.h b/tools/testing/selftests/kvm/include/arm64/nested.h
new file mode 100644
index 000000000000..739ff2ee0161
--- /dev/null
+++ b/tools/testing/selftests/kvm/include/arm64/nested.h

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * ARM64 Nested virtualization defines
+ */
+
+#ifndef SELFTEST_KVM_NESTED_H
+#define SELFTEST_KVM_NESTED_H
+
+void prepare_l2_stack(struct kvm_vm *vm, struct kvm_vcpu *vcpu);
+void prepare_hyp_state(struct kvm_vm *vm, struct kvm_vcpu *vcpu);
+void prepare_eret_destination(struct kvm_vm *vm, struct kvm_vcpu *vcpu, void *l2_pc);
+void prepare_nested_sync_handler(struct kvm_vm *vm, struct kvm_vcpu *vcpu);
+
+void run_l2(void);
+void after_hvc(void);
+void do_hvc(void);
+
+#endif /* SELFTEST_KVM_NESTED_H */

diff --git a/tools/testing/selftests/kvm/lib/arm64/nested.c b/tools/testing/selftests/kvm/lib/arm64/nested.c
new file mode 100644
index 000000000000..111d02f44cfe
--- /dev/null
+++ b/tools/testing/selftests/kvm/lib/arm64/nested.c

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * ARM64 Nested virtualization helpers
+ */
+
+#include "kvm_util.h"
+#include "nested.h"
+#include "processor.h"
+#include "test_util.h"
+
+#include <asm/sysreg.h>
+
+static void hvc_handler(struct ex_regs *regs)
+{
+	GUEST_ASSERT_EQ(get_current_el(), 2);
+	GUEST_PRINTF("hvc handler\n");
+	regs->pstate = PSR_MODE_EL2h | PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT;
+	regs->pc = (u64)after_hvc;
+}
+
+void prepare_l2_stack(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
+{
+	size_t l2_stack_size;
+	uint64_t l2_stack_paddr;
+
+	l2_stack_size = vm->page_size == 4096 ? DEFAULT_STACK_PGS * vm->page_size :
+					 vm->page_size;

Please use symbolic constants. Also, this looks wrong if the default
stack size is 32k and the page size is 16k. You probably want to
express a stack size directly, rather than a number of pages.

Makes sense, will fix the size of the stack.

quoted

+	l2_stack_paddr = __vm_phy_pages_alloc(vm, l2_stack_size / vm->page_size,
+					      0, 0, false);
+	vcpu_set_reg(vcpu, ARM64_CORE_REG(sp_el1), l2_stack_paddr + l2_stack_size);
+}
+
+void prepare_hyp_state(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
+{
+	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_HCR_EL2), HCR_EL2_RW);

Surely the E2H value matters. Or are you planning to only run this on
configuration that hardcode E2H==0? That'd be pretty limiting.

Yes it does matter, I was tunnel-visioned in trying to make L1 <-> L2
transition work with the bare minimum, and missed what we will want in
the future.

quoted

+}
+
+void prepare_eret_destination(struct kvm_vm *vm, struct kvm_vcpu *vcpu, void *l2_pc)
+{
+	vm_paddr_t do_hvc_paddr = addr_gva2gpa(vm, (vm_vaddr_t)do_hvc);
+	vm_paddr_t l2_pc_paddr = addr_gva2gpa(vm, (vm_vaddr_t)l2_pc);
+
+	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_SPSR_EL2), PSR_MODE_EL1h |
+							    PSR_D_BIT     |
+							    PSR_A_BIT     |
+							    PSR_I_BIT     |
+							    PSR_F_BIT);
+	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_ELR_EL2), l2_pc_paddr);
+	/* HACK: use TPIDR_EL2 to pass address, see run_l2() in nested_asm.S */
+	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_TPIDR_EL2), do_hvc_paddr);
+}
+
+void prepare_nested_sync_handler(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
+{
+	if (!vm->handlers) {
+		vm_init_descriptor_tables(vm);
+		vcpu_init_descriptor_tables(vcpu);
+	}
+	vm_install_sync_handler(vm, VECTOR_SYNC_LOWER_64,
+				ESR_ELx_EC_HVC64, hvc_handler);
+}

diff --git a/tools/testing/selftests/kvm/lib/arm64/nested_asm.S b/tools/testing/selftests/kvm/lib/arm64/nested_asm.S
new file mode 100644
index 000000000000..4ecf2d510a6f
--- /dev/null
+++ b/tools/testing/selftests/kvm/lib/arm64/nested_asm.S

@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * ARM64 Nested virtualization assembly helpers
+ */
+
+.globl run_l2
+.globl after_hvc
+.globl do_hvc
+run_l2:
+	/*
+	 * At this point TPIDR_EL2 will contain the gpa of do_hvc from
+	 * prepare_eret_destination(). gpa of do_hvc have to be passed in
+	 * because we want L2 to issue an hvc after it returns from the user
+	 * passed function. In order for that to happen the lr must be
+	 * controlled, which at this point holds the value of the address of
+	 * the next instruction after this run_l2() call, which is not useful
+	 * for L2. Additionally, L1 can't translate gva into gpa, so we can't
+	 * calculate it here.
+	 *
+	 * So first save lr, then move TPIDR_EL2 to lr so when the user supplied
+	 * L2 function returns, L2 jumps to do_hvc and let the L1 hvc handler
+	 * take control. This implies we expect the L2 code to preserve lr and
+	 * calls a regular ret in the end, which is true for normal C functions.
+	 * The hvc handler will jump back to after_hvc when finished, and lr
+	 * will be restored and we can return run_l2().
+	 */
+	stp	x29, lr, [sp, #-16]!
+	mrs	x0, tpidr_el2
+	mov	lr, x0
+	eret
+after_hvc:
+	ldp	x29, lr, [sp], #16
+	ret
+do_hvc:
+	hvc #0

This probably works for a single instruction L2 guest, but not having
any save/restore of the L2 context makes it hard to build anything on
top of this.

Agreed, we need L2 save/restore to meaningfully test NV.

Thanks,
Wei-Lin Chang

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.