Re: [PATCH v4 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi Vaibhav,

Thanks for reviewing this patch. Please find my response below.

On 2026/06/19 11:44 AM, Vaibhav Jain wrote:
> Hi Amit,
> 
> Thanks for the patch and incorporating V3 review comments. Further
> review comments inline below:
> 
> Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 
> > Add documentation for the KVM_PPC_GET_COMPAT_CAPS ioctl to the KVM API
> > documentation.
> >
> > The ioctl exposes host processor compatibility modes supported for
> > nested KVM guests on PowerPC systems. The documentation includes
> > comprehensive error code descriptions, structure field definitions
> > including the size field for forward compatibility, and KVM-specific
> > capability bit constants.
> >
> > Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> > ---
> >  Documentation/virt/kvm/api.rst | 47 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 47 insertions(+)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 52bbbb553ce1..ba6feba74d7d 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6553,6 +6553,53 @@ KVM_S390_KEYOP_SSKE
> >    Sets the storage key for the guest address ``guest_addr`` to the key
> >    specified in ``key``, returning the previous value in ``key``.
> >  
> > +4.145 KVM_PPC_GET_COMPAT_CAPS
> > +-----------------------------
> > +:Capability: KVM_CAP_PPC_COMPAT_CAPS
> > +:Architectures: powerpc
> > +:Type: vm ioctl
> > +:Parameters: struct kvm_ppc_compat_caps (out)
> > +:Returns: 0 on success, negative value on failure
> > +
> > +Errors include:
> > +
> > +  ======== ============================================================
> > +  EFAULT   if ``struct kvm_ppc_compat_caps`` cannot be read from or
> > +           written to userspace
> > +  EINVAL   if the ``size`` field is smaller than the current structure
> > +           size, or if the backend implementation fails to retrieve or
> > +           map CPU compatibility capabilities
> > +  ENOTTY   if the backend does not implement the ``get_compat_caps``
> > +           operation (e.g., on non-pseries platforms or when the
> > +           required KVM operations are not available)
> > +  ======== ============================================================
> > +
> > +IBM POWER system server-based processors provide a compatibility mode feature
> > +where an Nth generation processor can operate in modes consistent with earlier
> > +generations such as (N-1) and (N-2).
> > +
> > +This ioctl provides userspace with information about the CPU compatibility modes
> > +supported by the current host processor for booting the nested KVM guests on
> > +PowerNV (KVM nested APIv1) and PowerVM (KVM nested APIv2) platforms.
> > +
> 
> Please add a detail on how returned 'size' field can be less than what
> the userspace has sent and how it should be handled.

Good point! However, with the suggested change from patch-1 (using
strict equality check `user_size != sizeof(host_caps)`, the returned
size will always equal what userspace sent, because we reject any size
mismatch with -EINVAL.

I'll update the documentation to clarify the strict size matching
behavior:

- Userspace must set size to sizeof(struct kvm_ppc_compat_caps) before
  the ioctl
- The kernel validates this with strict equality (!= check)
- Returns -EINVAL if size doesn't match
- On success, the returned size will always equal what was sent

This should eliminates the forward-compatibility scenario where returned
size could differ from sent size.

Thank,
Amit

> 
> > +::
> > +
> > +  struct kvm_ppc_compat_caps {
> > +	__u64	flags;			/* Reserved for future use */
> > +	__u64	size;			/* Size of this structure */
> > +	__u64	compat_capabilities;	/* Capabilities supported by the host */
> > +  };
> > +
> > +The ``compat_capabilities`` bit field describes the processor compatibility
> > +modes supported by the host. For example, the following bits indicate support
> > +for specific processor modes.
> > +
> > +::
> > +
> > +  KVM_PPC_COMPAT_CAP_POWER9  (bit 1): KVM guests can run in Power9 processor mode
> > +  KVM_PPC_COMPAT_CAP_POWER10 (bit 2): KVM guests can run in Power10 processor mode
> > +  KVM_PPC_COMPAT_CAP_POWER11 (bit 3): KVM guests can run in Power11 processor mode
> > +
> >  .. _kvm_run:
> >  
> >  5. The kvm_run structure
> > -- 
> > 2.50.1 (Apple Git-155)
> >
> 
> -- 
> Cheers
> ~ Vaibhav