Re: [PATCH v2 3/4] KVM: arm64: sefltests: Add basic NV selftest

From: Wei-Lin Chang

Date: Tue Apr 14 2026 - 06:25:55 EST

On Tue, Apr 14, 2026 at 06:31:22AM +0900, Itaru Kitayama wrote:
> On Mon, Apr 13, 2026 at 10:18:42AM +0100, Wei-Lin Chang wrote:
> > Hi Itaru,
> >
> > On Mon, Apr 13, 2026 at 08:19:25AM +0900, Itaru Kitayama wrote:
> > > On Sun, Apr 12, 2026 at 03:22:15PM +0100, Wei-Lin Chang wrote:
> > > > This selftest simply starts an L1, which starts its own guest (L2). L2
> > > > runs without stage-1 and 2 translations, it calls an HVC to jump back
> > > > to L1.
> > >
> > > How do you disable both the nested guest (L2)'s MMU and stage 2
> > > translations?
> >
> > Guest stage-2 is disabled by not setting HCR_EL2.VM in prepare_hyp(),
> > and stage-1 is disabled by not writing to SCTLR_EL12 in init_vcpu(),
> > effectively using the default value set by L0. However since SCTLR_EL1
> > has many architecturally UNKNOWN bits (including SCTLR_EL1.M), it should
> > be better to write a value before running L2 I suppose...
>
> Thanks. What do you think of using copy_el2_to_el1() macro in at.c, so we
> can prepare in guest_code() to manipulate the SCTLR_EL12 System register
> with the sensible programmed values?

Yes, using copy_el2_to_el1() can give us an L2 stage-1 that is identical
to the L1's stage-1. But what I was considering was if guest stage-2 is
enabled (which we plan to implement), then those stage-1 page tables
will have to be mapped for L2, and its base address translated to L2IPA.
It's doable but seems like extra complexity when stage-1 is not so
interesting for KVM (except for AT?), it lets the guest do whatever it
likes and let the hardware do the translation.

Let me know if you have reasons to want stage-1 for L2, there could be
something I should consider but did not.

Thanks,
Wei-Lin Chang

>
> Itaru.
>
> >
> > Thanks,
> > Wei-Lin Chang
> >
> > >
> > > Itaru.
> > >
> > > >
> > > > Signed-off-by: Wei-Lin Chang <weilin.chang@xxxxxxx>
> > > > ---
> > > > tools/testing/selftests/kvm/Makefile.kvm | 1 +
> > > > .../selftests/kvm/arm64/hello_nested.c | 103 ++++++++++++++++++
> > > > 2 files changed, 104 insertions(+)
> > > > create mode 100644 tools/testing/selftests/kvm/arm64/hello_nested.c
> > > >
> > > > diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> > > > index 3dc3e39f7025..e8c108e0c487 100644
> > > > --- a/tools/testing/selftests/kvm/Makefile.kvm
> > > > +++ b/tools/testing/selftests/kvm/Makefile.kvm
> > > > @@ -168,6 +168,7 @@ TEST_GEN_PROGS_arm64 += arm64/arch_timer_edge_cases
> > > > TEST_GEN_PROGS_arm64 += arm64/at
> > > > TEST_GEN_PROGS_arm64 += arm64/debug-exceptions
> > > > TEST_GEN_PROGS_arm64 += arm64/hello_el2
> > > > +TEST_GEN_PROGS_arm64 += arm64/hello_nested
> > > > TEST_GEN_PROGS_arm64 += arm64/host_sve
> > > > TEST_GEN_PROGS_arm64 += arm64/hypercalls
> > > > TEST_GEN_PROGS_arm64 += arm64/external_aborts
> > > > diff --git a/tools/testing/selftests/kvm/arm64/hello_nested.c b/tools/testing/selftests/kvm/arm64/hello_nested.c
> > > > new file mode 100644
> > > > index 000000000000..97387e4697b3
> > > > --- /dev/null
> > > > +++ b/tools/testing/selftests/kvm/arm64/hello_nested.c
> > > > @@ -0,0 +1,103 @@
> > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > +/*
> > > > + * hello_nested - Go from vEL2 to EL1 then back
> > > > + */
> > > > +
> > > > +#include "nested.h"
> > > > +#include "processor.h"
> > > > +#include "test_util.h"
> > > > +#include "ucall.h"
> > > > +
> > > > +#define XLATE2GPA (0xABCD)
> > > > +#define L2STACKSZ (0x100)
> > > > +
> > > > +/*
> > > > + * TPIDR_EL2 is used to store vcpu id, so save and restore it.
> > > > + */
> > > > +static vm_paddr_t ucall_translate_to_gpa(void *gva)
> > > > +{
> > > > + vm_paddr_t gpa;
> > > > + u64 vcpu_id = read_sysreg(tpidr_el2);
> > > > +
> > > > + GUEST_SYNC2(XLATE2GPA, gva);
> > > > +
> > > > + /* get the result from userspace */
> > > > + gpa = read_sysreg(tpidr_el2);
> > > > +
> > > > + write_sysreg(vcpu_id, tpidr_el2);
> > > > +
> > > > + return gpa;
> > > > +}
> > > > +
> > > > +static void l2_guest_code(void)
> > > > +{
> > > > + do_hvc();
> > > > +}
> > > > +
> > > > +static void guest_code(void)
> > > > +{
> > > > + struct vcpu vcpu;
> > > > + struct hyp_data hyp_data;
> > > > + int ret;
> > > > + vm_paddr_t l2_pc, l2_stack_top;
> > > > + /* force 16-byte alignment for the stack pointer */
> > > > + u8 l2_stack[L2STACKSZ] __attribute__((aligned(16)));
> > > > +
> > > > + GUEST_ASSERT_EQ(get_current_el(), 2);
> > > > + GUEST_PRINTF("vEL2 entry\n");
> > > > +
> > > > + l2_pc = ucall_translate_to_gpa(l2_guest_code);
> > > > + l2_stack_top = ucall_translate_to_gpa(&l2_stack[L2STACKSZ]);
> > > > +
> > > > + init_vcpu(&vcpu, l2_pc, l2_stack_top);
> > > > + prepare_hyp();
> > > > +
> > > > + ret = run_l2(&vcpu, &hyp_data);
> > > > + GUEST_ASSERT_EQ(ret, ARM_EXCEPTION_TRAP);
> > > > + GUEST_DONE();
> > > > +}
> > > > +
> > > > +int main(void)
> > > > +{
> > > > + struct kvm_vcpu_init init;
> > > > + struct kvm_vcpu *vcpu;
> > > > + struct kvm_vm *vm;
> > > > + struct ucall uc;
> > > > + vm_paddr_t gpa;
> > > > +
> > > > + TEST_REQUIRE(kvm_check_cap(KVM_CAP_ARM_EL2));
> > > > + 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);
> > > > +
> > > > + while (true) {
> > > > + vcpu_run(vcpu);
> > > > +
> > > > + switch (get_ucall(vcpu, &uc)) {
> > > > + case UCALL_SYNC:
> > > > + if (uc.args[0] == XLATE2GPA) {
> > > > + gpa = addr_gva2gpa(vm, (vm_vaddr_t)uc.args[1]);
> > > > + vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_TPIDR_EL2), gpa);
> > > > + }
> > > > + break;
> > > > + case UCALL_PRINTF:
> > > > + pr_info("%s", uc.buffer);
> > > > + break;
> > > > + case UCALL_DONE:
> > > > + pr_info("DONE!\n");
> > > > + goto end;
> > > > + case UCALL_ABORT:
> > > > + REPORT_GUEST_ASSERT(uc);
> > > > + fallthrough;
> > > > + default:
> > > > + TEST_FAIL("Unhandled ucall: %ld\n", uc.cmd);
> > > > + }
> > > > + }
> > > > +
> > > > +end:
> > > > + kvm_vm_free(vm);
> > > > + return 0;
> > > > +}
> > > > --
> > > > 2.43.0
> > > >