Re: [PATCH v8 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl

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.

So as for this patch, the changes looks good to me. Please feel free to
add:

Reviewed-by: Ritesh Harjani (IBM) <[email protected]>

>  arch/powerpc/include/asm/kvm_ppc.h  |  1 +
>  arch/powerpc/include/uapi/asm/kvm.h |  8 +++
>  arch/powerpc/kvm/powerpc.c          | 78 +++++++++++++++++++++++++++++
>  include/uapi/linux/kvm.h            |  3 ++
>  4 files changed, 90 insertions(+)