[PATCH kvmtool v3 2/6] arm64: Initial nested virt support

Andre Przywara
2025-07-29T09:58:01Z

The ARMv8.3 architecture update includes support for nested
virtualization. Allow the user to specify "--nested" to start a guest in
(virtual) EL2 instead of EL1.
This will also change the PSCI conduit from HVC to SMC in the device
tree.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
---
arm64/fdt.c | 5 ++++-
arm64/include/kvm/kvm-config-arch.h | 5 ++++-
arm64/kvm-cpu.c | 12 +++++++++++-
3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arm64/fdt.c b/arm64/fdt.c
index df7775876..98f1dd9d4 100644
--- a/arm64/fdt.c
+++ b/arm64/fdt.c
@@ -205,7 +205,10 @@ static int setup_fdt(struct kvm *kvm)
_FDT(fdt_property_string(fdt, "compatible", "arm,psci"));
fns = &psci_0_1_fns;
}
- _FDT(fdt_property_string(fdt, "method", "hvc"));
+ if (kvm->cfg.arch.nested_virt)
+ _FDT(fdt_property_string(fdt, "method", "smc"));
+ else
+ _FDT(fdt_property_string(fdt, "method", "hvc"));
_FDT(fdt_property_cell(fdt, "cpu_suspend", fns->cpu_suspend));
_FDT(fdt_property_cell(fdt, "cpu_off", fns->cpu_off));
_FDT(fdt_property_cell(fdt, "cpu_on", fns->cpu_on));
diff --git a/arm64/include/kvm/kvm-config-arch.h b/arm64/include/kvm/kvm-config-arch.h
index ee031f010..a1dac28e6 100644
--- a/arm64/include/kvm/kvm-config-arch.h
+++ b/arm64/include/kvm/kvm-config-arch.h
@@ -10,6 +10,7 @@ struct kvm_config_arch {
bool aarch32_guest;
bool has_pmuv3;
bool mte_disabled;
+ bool nested_virt;
u64 kaslr_seed;
enum irqchip_type irqchip;
u64 fw_addr;
@@ -57,6 +58,8 @@ int sve_vl_parser(const struct option *opt, const char *arg, int unset);
"Type of interrupt controller to emulate in the guest", \
irqchip_parser, NULL), \
OPT_U64('\0', "firmware-address", &(cfg)->fw_addr, \
- "Address where firmware should be loaded"),
+ "Address where firmware should be loaded"), \
+ OPT_BOOLEAN('\0', "nested", &(cfg)->nested_virt, \
+ "Start VCPUs in EL2 (for nested virt)"),

#endif /* ARM_COMMON__KVM_CONFIG_ARCH_H */
diff --git a/arm64/kvm-cpu.c b/arm64/kvm-cpu.c
index 94c08a4d7..42dc11dad 100644
--- a/arm64/kvm-cpu.c
+++ b/arm64/kvm-cpu.c
@@ -71,6 +71,12 @@ static void kvm_cpu__select_features(struct kvm *kvm, struct kvm_vcpu_init *init
/* Enable SVE if available */
if (kvm__supports_extension(kvm, KVM_CAP_ARM_SVE))
init->features[0] |= 1UL << KVM_ARM_VCPU_SVE;
+
+ if (kvm->cfg.arch.nested_virt) {
+ if (!kvm__supports_extension(kvm, KVM_CAP_ARM_EL2))
+ die("EL2 (nested virt) is not supported");
+ init->features[0] |= 1UL << KVM_ARM_VCPU_HAS_EL2;
+ }
}

static int vcpu_configure_sve(struct kvm_cpu *vcpu)
@@ -313,7 +319,11 @@ static void reset_vcpu_aarch64(struct kvm_cpu *vcpu)
reg.addr = (u64)&data;

/* pstate = all interrupts masked */
- data = PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT | PSR_MODE_EL1h;
+ data = PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT;
+ if (vcpu->kvm->cfg.arch.nested_virt)
+ data |= PSR_MODE_EL2h;
+ else
+ data |= PSR_MODE_EL1h;
reg.id = ARM64_CORE_REG(regs.pstate);
if (ioctl(vcpu->vcpu_fd, KVM_SET_ONE_REG, &reg) < 0)
die_perror("KVM_SET_ONE_REG failed (spsr[EL1])");
--
2.25.1