Re: [PATCH v7 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

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 served from
> the cached nested_capabilities value (populated at module init via
> kvmhv_nested_init() using 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.
>
> The mapping is cumulative: a system running at a given compatibility
> level is assumed to also support older generations down the supported
> chain. Note that unlike KVM on PowerVM (nested API v2), KVM on PowerNV
> currently does not strictly enforce older generation compatibility modes
> for nested guests - the reported capabilities reflect what the host CPU
> can present, not what the hypervisor independently validates.
>
> Introduce a helper kvmppc_map_compat_capabilities() to translate CPU
> version values into KVM_PPC_COMPAT_CAP bits using a fallthrough switch,
> 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
> KVM on PowerVM and on PowerNV platforms via the KVM_PPC_GET_COMPAT_CAPS
> ioctl.
>
> Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> Tested-by: Gautam Menghani <gautam@linux.ibm.com>
> Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
> Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 152cd08a5b38..4d0307f55e06 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -6523,20 +6523,56 @@ static bool kvmppc_hash_v3_possible(void)
>  	return true;
>  }
>  
> +static int kvmppc_map_compat_capabilities(u32 cpu_version,
> +					  unsigned long *capabilities)
> +{
> +	switch (cpu_version) {
> +	case PVR_ARCH_31_P11:
> +		*capabilities |= KVM_PPC_COMPAT_CAP_POWER11;
> +		fallthrough;
> +	case PVR_ARCH_31:
> +		*capabilities |= KVM_PPC_COMPAT_CAP_POWER10;
> +		fallthrough;
> +	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;
>  	long rc = -EINVAL;
> +	u32 cpu_version = 0;
>  
>  	if (kvmhv_on_pseries()) {
>  		if (kvmhv_is_nestedv2()) {
>  			WARN_ON_ONCE(!nested_capabilities);
>  			capabilities = nested_capabilities;
>  			rc = 0;
> +		} else {
> +			for_each_node_by_type(np, "cpu") {
> +				if (!of_property_read_u32(np, "cpu-version",
> +							  &cpu_version)) {
> +					of_node_put(np);
> +					break;
> +				}
> +			}

What happens when we don't have "cpu-version" DT property?
Check this commit
5a61ef74f269f2 ("powerpc/64s: Support new device tree binding for discovering CPU features")

And also why do we need to parse the DT properties again?
Shouldn't we check something like this?

	if (cpu_has_feature(CPU_FTR_P11_PVR))
		capabilities |= KVM_PPC_COMPAT_CAP_POWER11;
	if (cpu_has_feature(CPU_FTR_ARCH_31))
		capabilities |= KVM_PPC_COMPAT_CAP_POWER10;
	if (cpu_has_feature(CPU_FTR_ARCH_300))
		capabilities |= KVM_PPC_COMPAT_CAP_POWER9;
	

-ritesh

> +			if (!cpu_version)
> +				return -EINVAL;
> +			rc = kvmppc_map_compat_capabilities(cpu_version,
> +							    &capabilities);
>  		}
>  	}
>  
> +	if (rc < 0)
> +		return rc;
> +
>  	host_caps->compat_capabilities = capabilities & KVM_PPC_COMPAT_BITMASK;
>  
>  	return rc;
> -- 
> 2.50.1 (Apple Git-155)