Re: [PATCH 4/6] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM

[Harsh Prateek Bora <harshpb@linux.ibm.com>]

On 30/04/26 11:19 am, Amit Machhiwal wrote:
> On POWER systems, the host CPU may run in a compatibility mode (e.g., a
> Power11 processor operating in Power10 compatibility mode). In such
> cases, the effective CPU level exposed to guests differs from the
> physical processor generation.
> 
> When running nested KVM guests, QEMU derives the host CPU type using
> mfpvr(), which reflects the physical processor version. This can result
> in a mismatch between the CPU model selected by QEMU and the
> compatibility mode enforced by the host, leading to guest boot failures.
> 
> For example, booting a nested guest on a Power11 LPAR configured in
> Power10 compatibility mode fails with:
> 
>    KVM-NESTEDv2: couldn't set guest wide elements
>    [..KVM reg dump..]
> 
> This occurs because QEMU selects a CPU model corresponding to the
> physical processor (via mfpvr()), while the host operates in a lower
> compatibility mode. As a result, KVM rejects the requested compatibility
> level during guest initialization.
> 
> Add support for retrieving host CPU compatibility capabilities for
> nested guests on PowerVM (PAPR nested API v2). The hypervisor provides
> the effective compatibility levels via the H_GUEST_GET_CAPABILITIES
> hcall, which reflects the processor modes negotiated between the Power
> hypervisor (L0) and the host partition (L1).
> 
> On pseries systems, obtain the capability bitmap using
> plpar_guest_get_capabilities() and return it via struct
> kvm_ppc_compat_caps. This information is then exposed to userspace
> through the KVM_PPC_GET_COMPAT_CAPS ioctl.
> 
> Hook the implementation into the Book3S HV kvmppc_ops so that it can be
> invoked by the generic KVM ioctl handling code.
> 
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>   arch/powerpc/kvm/book3s_hv.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 948c6b099a29..d602d90111d1 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -6516,6 +6516,22 @@ static bool kvmppc_hash_v3_possible(void)
>   	return true;
>   }
>   
> +
> +static int kvmppc_get_compat_cpu_caps(struct kvm_ppc_compat_caps *host_caps)
> +{
> +
> +	unsigned long capabilities = 0;
> +	long rc = -EINVAL;
> +
> +	if (kvmhv_on_pseries()) {
> +		if (kvmhv_is_nestedv2())

If this is only intended for nested APIv2, it should reflect in commit 
log/title, otherwise, we need to return success with capabilities set to 
0 for non-nestedv2 case.

Also, better to have combined check for pseries and nestedv2 if we are 
not handling other cases.

> +			rc = plpar_guest_get_capabilities(0, &capabilities);
> +		host_caps->compat_capabilities = capabilities;

Do we need to take care of PowerNV case ?

> +	}
> +
> +	return rc;
> +}
> +
>   static struct kvmppc_ops kvm_ops_hv = {
>   	.get_sregs = kvm_arch_vcpu_ioctl_get_sregs_hv,
>   	.set_sregs = kvm_arch_vcpu_ioctl_set_sregs_hv,
> @@ -6558,6 +6574,7 @@ static struct kvmppc_ops kvm_ops_hv = {
>   	.hash_v3_possible = kvmppc_hash_v3_possible,
>   	.create_vcpu_debugfs = kvmppc_arch_create_vcpu_debugfs_hv,
>   	.create_vm_debugfs = kvmppc_arch_create_vm_debugfs_hv,
> +	.get_compat_cpu_ver = kvmppc_get_compat_cpu_caps,
>   };
>   
>   static int kvm_init_subcore_bitmap(void)