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

Alexandru Elisei
2025-08-04T14:41:53Z

Hi Andre,

On Tue, Jul 29, 2025 at 10:57:41AM +0100, Andre Przywara wrote:
> 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, \

--nested sounds a bit vague (what if KVM decides to nest something else in the
future?) and the variable that keeps track of the parameter is called
'nested_virt'. Is it too late to rename --nested to --nested-virt for
consistency and better clarity?

> + "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");

Marc pointed out that KVM_CAP_ARM_EL2 does more that enable EL2, it exposes
nested virtualization to a level 1 guest. How about rewording the error message
to something like this: "Nested virt is not supported"?

Thanks,
Alex

> + 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
>