Re: [PATCH 04/59] KVM: arm64: nv: Introduce nested virtualization VCPU feature

Marc Zyngier
2019-07-03T11:54:06Z

On 24/06/2019 12:28, Dave Martin wrote:
> On Fri, Jun 21, 2019 at 10:37:48AM +0100, Marc Zyngier wrote:
>> From: Christoffer Dall <christoffer.dall@arm.com>
>>
>> Introduce the feature bit and a primitive that checks if the feature is
>> set behind a static key check based on the cpus_have_const_cap check.
>>
>> Checking nested_virt_in_use() on systems without nested virt enabled
>> should have neglgible overhead.
>>
>> We don't yet allow userspace to actually set this feature.
>>
>> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>> arch/arm/include/asm/kvm_nested.h | 9 +++++++++
>> arch/arm64/include/asm/kvm_nested.h | 13 +++++++++++++
>> arch/arm64/include/uapi/asm/kvm.h | 1 +
>> 3 files changed, 23 insertions(+)
>> create mode 100644 arch/arm/include/asm/kvm_nested.h
>> create mode 100644 arch/arm64/include/asm/kvm_nested.h
>>
>> diff --git a/arch/arm/include/asm/kvm_nested.h b/arch/arm/include/asm/kvm_nested.h
>> new file mode 100644
>> index 000000000000..124ff6445f8f
>> --- /dev/null
>> +++ b/arch/arm/include/asm/kvm_nested.h
>> @@ -0,0 +1,9 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef __ARM_KVM_NESTED_H
>> +#define __ARM_KVM_NESTED_H
>> +
>> +#include <linux/kvm_host.h>
>> +
>> +static inline bool nested_virt_in_use(const struct kvm_vcpu *vcpu) { return false; }
>> +
>> +#endif /* __ARM_KVM_NESTED_H */
>> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
>> new file mode 100644
>> index 000000000000..8a3d121a0b42
>> --- /dev/null
>> +++ b/arch/arm64/include/asm/kvm_nested.h
>> @@ -0,0 +1,13 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef __ARM64_KVM_NESTED_H
>> +#define __ARM64_KVM_NESTED_H
>> +
>> +#include <linux/kvm_host.h>
>> +
>> +static inline bool nested_virt_in_use(const struct kvm_vcpu *vcpu)
>> +{
>> + return cpus_have_const_cap(ARM64_HAS_NESTED_VIRT) &&
>> + test_bit(KVM_ARM_VCPU_NESTED_VIRT, vcpu->arch.features);
>> +}
>> +
>> +#endif /* __ARM64_KVM_NESTED_H */
>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
>> index d819a3e8b552..563e2a8bae93 100644
>> --- a/arch/arm64/include/uapi/asm/kvm.h
>> +++ b/arch/arm64/include/uapi/asm/kvm.h
>> @@ -106,6 +106,7 @@ struct kvm_regs {
>> #define KVM_ARM_VCPU_SVE 4 /* enable SVE for this CPU */
>> #define KVM_ARM_VCPU_PTRAUTH_ADDRESS 5 /* VCPU uses address authentication */
>> #define KVM_ARM_VCPU_PTRAUTH_GENERIC 6 /* VCPU uses generic authentication */
>> +#define KVM_ARM_VCPU_NESTED_VIRT 7 /* Support nested virtualization */
>
> This seems weirdly named:
>
> Isn't the feature we're exposing here really EL2? In that case, the
> feature the guest gets with this flag enabled is plain virtualisation,
> possibly with the option to nest further.
>
> Does the guest also get nested virt (i.e., recursively nested virt from
> the host's PoV) as a side effect, or would require an explicit extra
> flag?

So far, there is no extra flag to describe further nesting, and it
directly comes from EL2 being emulated. I don't mind renaming this to
KVM_ARM_VCPU_HAS_EL2, or something similar... Whether we want userspace
to control the exposure of the nesting capability (i.e. EL2 with
ARMv8.3-NV) is another question.

Thanks,

M.
--
Jazz is not dead. It just smells funny...