Re: [PATCH v4 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV

[Vaibhav Jain <vaibhav@linux.ibm.com>]

Hi Amit.

Thanks for the patch and incorporating V3 review comments. Further
review comments inline below:

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> Currently, when booting a compatibility-mode KVM guest (L1) on a PowerNV
> hypervisor (L0), the guest runs with the expected processor
> compatibility level. However, when booting a nested KVM guest (L2)
> inside the L1, QEMU derives the CPU model from the raw host PVR and
> attempts to run the nested guest at that level, instead of honoring the
> compatibility mode of the L1.
>
> Extend host CPU compatibility capability reporting to support nested
> virtualization on PowerNV systems (PAPR nested API v1).
>
> For nested API v2 (PowerVM), compatibility capabilities are obtained
> from the hypervisor via the H_GUEST_GET_CAPABILITIES hcall. This
> information is not available on PowerNV systems.
>
> For nested API v1, derive the compatibility capabilities from the L1
> guest by reading the "cpu-version" property from the device tree, which
> reflects the effective (logical) processor compatibility level. Map this
> value to the corresponding compatibility capability bitmap using
> KVM-specific constants.
>
> Introduce a helper to translate CPU version values into KVM_PPC_COMPAT_CAP
> bits and integrate it into kvmppc_get_compat_caps(). The implementation
> applies masking to ensure only supported processor modes are exposed.
>
> This allows userspace to query host CPU compatibility modes on both
> PowerVM and PowerNV platforms via the KVM_PPC_GET_COMPAT_CAPS ioctl.
>
> Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 37 +++++++++++++++++++++++++++++++++++-
>  1 file changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index f674386df62c..375e7a7fa9f8 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -6523,15 +6523,50 @@ static bool kvmppc_hash_v3_possible(void)
>  	return true;
>  }
>  
> +static int kvmppc_map_compat_capabilities(const __be32 cpu_version,
> +				      unsigned long *capabilities)
> +{
> +	switch (cpu_version) {
> +	case PVR_ARCH_31_P11:
> +		*capabilities |= KVM_PPC_COMPAT_CAP_POWER11;
Do you need to do 'break' here instead of falling through. Since P11
host can support P10 and P9 compat modes

> +		break;
> +	case PVR_ARCH_31:
> +		*capabilities |= KVM_PPC_COMPAT_CAP_POWER10;
> +		break;
> +	case PVR_ARCH_300:
> +		*capabilities |= KVM_PPC_COMPAT_CAP_POWER9;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
>  
>  static int kvmppc_get_compat_caps(struct kvm_ppc_compat_caps *host_caps)
>  {
> +	struct device_node *np;
>  	unsigned long capabilities = 0;
> +	const __be32 *prop = NULL;
>  	long rc = -EINVAL;
> +	u32 cpu_version;
>  
>  	if (kvmhv_on_pseries()) {
> -		if (kvmhv_is_nestedv2())
> +		if (kvmhv_is_nestedv2()) {
>  			rc = plpar_guest_get_capabilities(0, &capabilities);
> +		} else {
> +			for_each_node_by_type(np, "cpu") {
> +				prop = of_get_property(np, "cpu-version", NULL);
> +				if (prop) {
> +					cpu_version = be32_to_cpup(prop);
> +					break;
> +				}
> +			}
> +			if (!prop)
> +				return -EINVAL;
> +			rc = kvmppc_map_compat_capabilities(cpu_version,
> +								&capabilities);
> +		}
should you check for 'rc' error here before assigning 'capabilities' to
'host_caps->compat_capabilities' . I understand it will be set to '0'
due to its initialization at the top of the function. But would be
better to make it more explicit

>  		host_caps->compat_capabilities = capabilities &
>  							KVM_PPC_COMPAT_BITMASK;
>  	}
> -- 
> 2.50.1 (Apple Git-155)
>
>

-- 
Cheers
~ Vaibhav