Re: [PATCH v2 2/4] sysctl: Make XEN_SYSCTL_topologyinfo sysctl a little more efficient

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

On 06/01/15 02:18, Boris Ostrovsky wrote:
> Instead of copying data for each field in xen_sysctl_topologyinfo separately
> put cpu/socket/node into a single structure and do a single copy for each
> processor.
>
> There is also no need to copy whole op to user at the end, max_cpu_index is
> sufficient
>
> Rename xen_sysctl_topologyinfo and XEN_SYSCTL_topologyinfo to reflect the fact
> that these are used for CPU topology. Subsequent patch will add support for
> PCI topology sysctl.
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

If we are going to change the hypercall, then can we see about making it
a stable interface (i.e. not a sysctl/domctl)?  There are non-toolstack
components which might want/need access to this information.  (i.e. I am
still looking for a reasonable way to get this information from Xen in
hwloc)

> <snip>
>
> +            if ( cpu_online(i) )
>              {
> -                uint32_t socket = cpu_online(i) ? cpu_to_socket(i) : ~0u;
> -                if ( copy_to_guest_offset(ti->cpu_to_socket, i, &socket, 1) )
> -                    break;
> +                cputopo.core = cpu_to_core(i);
> +                cputopo.socket = cpu_to_socket(i);
> +                cputopo.node = cpu_to_node(i);
>              }
> -            if ( !guest_handle_is_null(ti->cpu_to_node) )
> +            else
> +                cputopo.core = cputopo.socket =
> +                    cputopo.node = INVALID_TOPOLOGY_ID;
> +

In particular, can we fix this broken behaviour.  The cpu being online
or not has no bearing on whether Xen has topology information, and a
side effect is that when the cpu governer powers down a cpu, it
disappears from the reported topology.

~Andrew