On 2026/08/08 06:30 AM, Ritesh Harjani wrote: > Amit Machhiwal <[email protected]> writes: > > > Introduce a new capability and ioctl to expose CPU compatibility modes > > supported by the host processor for nested guests. > > > > On IBM POWER systems, newer processor generations (N) can operate in > > compatibility modes corresponding to earlier generations, like (N-1) and > > (N-2). This is particularly relevant for nested virtualization, where > > nested KVM guests may need to run with a specific processor compatibility > > level. > > > > Introduce KVM_CAP_PPC_COMPAT_CAPS capability and the corresponding > > KVM_PPC_GET_COMPAT_CAPS vm ioctl. The ioctl returns a bitmap describing > > the compatibility modes supported by the host in respective bit numbers, > > allowing userspace (e.g., QEMU) to select an appropriate compatibility > > level when configuring nested KVM guests. > > > > The ioctl handling is added in kvm_arch_vm_ioctl() and retrieves host > > CPU compatibility capabilities via a PowerPC-specific backend > > implementation when available. > > > > The struct kvm_ppc_compat_caps places the 'size' field first so it can > > be read alone via get_user() before copy_struct_from_user() is called, > > avoiding pointer arithmetic to locate the size field. > > > > The ioctl is defined using _IO so the ioctl number remains stable even if > > the struct grows in future versions. It uses copy_struct_from_user() and > > copy_struct_to_user() to provide forward- and backward-compatible > > extensibility: older userspace passing a smaller struct to a newer kernel > > gets zero-padded trailing fields. Newer userspace passing a larger struct to > > an older kernel (usize > ksize) succeeds if trailing bytes are zero (the > > kernel reports back min(usize, ksize) as the filled size); if trailing bytes > > are non-zero, the kernel writes back ksize into host_caps.size and returns > > -E2BIG so userspace can retry with the correct size. > > > > KVM_PPC_COMPAT_CAPS_SIZE_VER0 is defined as a frozen integer constant > > (24) marking the size of the initial struct version, used as the > > minimum floor for size field validation, similar to other versioned > > struct interfaces in the kernel. > > > > The 'flags' field is reserved for future use. The kernel rejects any > > call where flags is non-zero with -EINVAL, preventing garbage values > > from being baked into ABI permanently. > > > > The ioctl returns appropriate error codes: E2BIG if usize exceeds > > PAGE_SIZE, or if new userspace provides a larger struct with non-zero > > trailing bytes (with ksize written back into host_caps.size for the > > retry); EINVAL for an invalid size or non-zero reserved fields; EFAULT > > for failed copy operations; and ENOTTY if the backend doesn't implement > > get_compat_caps. > > > > Suggested-by: Vaibhav Jain <[email protected]> > > Tested-by: Gautam Menghani <[email protected]> > > Reviewed-by: Gautam Menghani <[email protected]> > > Tested-by: Anushree Mathur <[email protected]> > > Signed-off-by: Amit Machhiwal <[email protected]> > > --- > > Changes in this version: > > - Add PAGE_SIZE guard after get_user() to bound the check_zeroed_user() > > scan in the usize > ksize path [Ritesh] > > - Drop manual usize > sizeof(host_caps) pre-check; delegate entirely to > > copy_struct_from_user() which succeeds on zero trailing bytes and > > returns -E2BIG only on non-zero trailing bytes; handle -E2BIG with > > ksize writeback and -EFAULT escalation if put_user() fails [Ritesh] > > - Fix host_caps.size on success path: use min_t(u64, usize, > > sizeof(host_caps)) so new userspace with zero trailing bytes gets back > > the number of bytes the kernel actually populated, not usize [Ritesh] > > > > Thanks for addressing them! > The only remaining comments from Sashiko now are because, it cannot find > the implementation of ->get_compat_caps() since it is in the next patch.
Yes, correct. > > So as for this patch, the changes looks good to me. Please feel free to > add: > > Reviewed-by: Ritesh Harjani (IBM) <[email protected]> Thanks for the review, Ritesh! Will carry the Reviewed-by in v9. ~Amit
