Re: [PATCH v6 8/9] KVM: arm64: Check whether a VM IOCTL is allowed in pKVM

[Marc Zyngier <maz@kernel.org>]

On Thu, 11 Dec 2025 10:47:08 +0000,
Fuad Tabba <tabba@google.com> wrote:
> 
> Certain VM IOCTLs are tied to specific VM features. Since pKVM does not
> support all features, restrict which IOCTLs are allowed depending on
> whether the associated feature is supported.
> 
> Use the existing VM capability check as the source of truth to whether
> an IOCTL is allowed for a particular VM by mapping the IOCTLs with their
> associated capabilities.
> 
> Suggested-by: Oliver Upton <oupton@kernel.org>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  arch/arm64/include/asm/kvm_pkvm.h | 21 +++++++++++++++++++++
>  arch/arm64/kvm/arm.c              |  3 +++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_pkvm.h b/arch/arm64/include/asm/kvm_pkvm.h
> index 5b564576160d..0fa8c84816fd 100644
> --- a/arch/arm64/include/asm/kvm_pkvm.h
> +++ b/arch/arm64/include/asm/kvm_pkvm.h
> @@ -9,6 +9,7 @@
>  #include <linux/arm_ffa.h>
>  #include <linux/memblock.h>
>  #include <linux/scatterlist.h>
> +#include <asm/kvm_host.h>
>  #include <asm/kvm_pgtable.h>
>  
>  /* Maximum number of VMs that can co-exist under pKVM. */
> @@ -43,6 +44,7 @@ static inline bool kvm_pkvm_ext_allowed(struct kvm *kvm, long ext)
>  	case KVM_CAP_ARM_SVE:
>  	case KVM_CAP_ARM_PTRAUTH_ADDRESS:
>  	case KVM_CAP_ARM_PTRAUTH_GENERIC:
> +	case KVM_CAP_ARM_BASIC:

Can we instead rely on an existing VM capability? I'm not overly keen
exposing something new to userspace (KVM_CAP_ARM_BASIC) for something
that really is KVM's own internal problems.

Looking at the history, a bunch of things have always been present:
KVM_CAP_NR_VCPUS, for example. You could even #define
KVM_CAP_ARM_BASIC to that if you want.

Thanks,

	M.

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