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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/07/2015 12:04 PM, Andrew Cooper wrote:
> 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@xxxxxxxxxx>
> > Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> 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.

Currently the caller, in fact, can have either of three pointers 
(node_to_memsize, node_to_memfree or node_to_node_distance) as NULL and 
the hypervisor will fill whichever pointer is valid. Because I put the 
first two together into a struct we are already changing behavior in 
that regard. Not to mention that having all three as NULL now has new 
meaning as well.
I thought that either both pointers should be valid or neither. If 
people disagree I can change this.
-boris

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