Re: [PATCH v4 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM

[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:

> 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. The implementation defines KVM-specific capability
> constants (KVM_PPC_COMPAT_CAP_POWER9/10/11) and applies masking to ensure
> only supported processor modes are exposed to userspace. This information
> is then exposed 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.
>
> Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>  arch/powerpc/include/uapi/asm/kvm.h | 11 ++++++++++-
>  arch/powerpc/kvm/book3s_hv.c        | 17 +++++++++++++++++
>  2 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
> index 8a38be6c3b03..730488681443 100644
> --- a/arch/powerpc/include/uapi/asm/kvm.h
> +++ b/arch/powerpc/include/uapi/asm/kvm.h
> @@ -443,7 +443,16 @@ struct kvm_ppc_compat_caps {
>  	__u64	size;			/* Size of this structure */
>  	__u64	compat_capabilities;	/* Capabilities supported by the host */
>  };
> -
> +/*
> + * Capability bits for compat_capabilities field in kvm_ppc_compat_caps.
> + * These bits indicate which processor compatibility modes are supported.
> + */
> +#define KVM_PPC_COMPAT_CAP_POWER9	(1ULL << 62)
> +#define KVM_PPC_COMPAT_CAP_POWER10	(1ULL << 61)
> +#define KVM_PPC_COMPAT_CAP_POWER11	(1ULL << 60)
> +#define KVM_PPC_COMPAT_BITMASK		(KVM_PPC_COMPAT_CAP_POWER9 | \
> +					 KVM_PPC_COMPAT_CAP_POWER10 | \
> +					 KVM_PPC_COMPAT_CAP_POWER11)
>  /*
>   * Values for character and character_mask.
>   * These are identical to the values used by H_GET_CPU_CHARACTERISTICS.
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index f9380ef65750..f674386df62c 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -6523,6 +6523,22 @@ static bool kvmppc_hash_v3_possible(void)
>  	return true;
>  }
>  
> +
> +static int kvmppc_get_compat_caps(struct kvm_ppc_compat_caps *host_caps)
> +{
> +	unsigned long capabilities = 0;
> +	long rc = -EINVAL;
> +
> +	if (kvmhv_on_pseries()) {
> +		if (kvmhv_is_nestedv2())
> +			rc = plpar_guest_get_capabilities(0,
> &capabilities);
I think instead of making the hcall you should use the
'nested_capabilities' extern symbol as it would already the same
value. This symbol is already accessible in 'book3s_hv.c'

> +		host_caps->compat_capabilities = capabilities &
> +							KVM_PPC_COMPAT_BITMASK;
> +	}
> +
> +	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,
> @@ -6565,6 +6581,7 @@ 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_caps = kvmppc_get_compat_caps,
>  };
>  
>  static int kvm_init_subcore_bitmap(void)
> -- 
> 2.50.1 (Apple Git-155)
>
>

-- 
Cheers
~ Vaibhav