Re: [PATCH v9 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Premature advertisement of KVM_CAP_PPC_COMPAT_CAPS causes bisectability breakages.
--

--- Patch [1]: [PATCH v9 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl ---
commit 92bc8045e2354b233ee21f84717a2a67b2fadcac
Author: Amit Machhiwal <[email protected]>

KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl

This commit introduces a new capability, KVM_CAP_PPC_COMPAT_CAPS, and an
associated ioctl, KVM_PPC_GET_COMPAT_CAPS. It aims to allow userspace to
discover host processor compatibility modes for nested KVM guests.

> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 00302399fc37b..2fe4ea0ca4ff1 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
[ ... ]
> @@ -697,6 +697,13 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  			}
>  		}
>  		break;
> +#if defined(CONFIG_KVM_BOOK3S_HV_POSSIBLE)
> +	case KVM_CAP_PPC_COMPAT_CAPS:
> +		r = 0;
> +		if (hv_enabled && kvmhv_on_pseries())
> +			r = 1;
> +		break;
> +#endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */

[Severity: High]
Will advertising this capability here break bisectability?

If userspace probes for KVM_CAP_PPC_COMPAT_CAPS, it will be reported as
supported on pSeries. However, since the backend for get_compat_caps isn't
implemented in this commit, the newly added KVM_PPC_GET_COMPAT_CAPS ioctl
will always fail:

[ ... ]

> +		/* Reserved fields must be zero */
> +		r = -EINVAL;
> +		if (host_caps.flags)
> +			goto out;
> +
> +		r = -ENOTTY;
> +		if (!kvm->arch.kvm_ops->get_compat_caps)
> +			goto out;

Could this unexpected -ENOTTY failure cause VM initialization in userspace
tools like QEMU to abort when checking this capability during a bisect?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1