Re: [PATCH v6 1/5] sysctl: Make XEN_SYSCTL_numainfo a little more efficient

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 06/04/15 23:12, Boris Ostrovsky wrote:
> A number of changes to XEN_SYSCTL_numainfo interface:
>
> * Make sysctl NUMA topology query use fewer copies by combining some
>   fields into a single structure and copying distances for each node
>   in a single copy.
> * NULL meminfo and distance handles are a request for maximum number
>   of nodes (num_nodes). If those handles are valid and num_nodes is
>   is smaller than the number of nodes in the system then -ENOBUFS is
>   returned (and correct num_nodes is provided)
> * Instead of using max_node_index for passing number of nodes keep this
>   value in num_nodes: almost all uses of max_node_index required adding
>   or subtracting one to eventually get to number of nodes anyway.
> * Replace INVALID_NUMAINFO_ID with XEN_INVALID_MEM_SZ and add
>   XEN_INVALID_NODE_DIST.
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Acked-by: Ian Campbell <ian.campbell@citrix.com>

This subtly changes the behaviour of XEN_SYSCTL_numainfo with regards to
NULL guest handles.

Previously, a caller was able to select which information they wanted by
choosing which guest handles were non-NULL.

With the new semantics, the caller must pass both ni->meminfo and
ni->distance to get either bit of information.  Each
copy_to_guest_offset() should be gated on a !guest_handle_is_null() so a
caller can request meminfo information without distance information.

~Andrew