Re: [Qemu-devel] [PATCH 00/16 v4] target-i386: CPU hot-add with cpu-add QMP command

[Eduardo Habkost]

On Tue, Apr 16, 2013 at 11:43:42AM +0200, Igor Mammedov wrote:
[...]
> BTW:
>  You were advocating using APIC ID on device_add to identify CPU but Eduardo
>  would like avoid its usage on external interfaces.
>  It would be nice to have your opinion on subject.
>   relevant discussion threads are here:
>     1. http://lists.gnu.org/archive/html/qemu-devel/2013-04/msg02205.html
>     2. http://lists.gnu.org/archive/html/qemu-devel/2013-04/msg02274.html

Trying to summarize my concerns quickly:

 0) APIC IDs must be calculated by QEMU based on CPU topology and can't
    be chosen arbitrarily.
 1) Requiring APIC ID as the CPU ID requires a target-independent
    probing interface to find out which IDs are valid, and how they
    correspond to locations in the CPU topology. We don't have such
    interface yet.
    * This makes it impossible to use APIC IDs to identify CPUs in the
      command-line (before QMP is available), so we still need another
      kind of identifier (topology-based, probably).
 2) apic-id is not compatible with the existing IDs used for the "-numa"
    command-line option.

So, using APIC ID is possible, but I believe it introduces too much
unnecessary complexity in the external interface. The day we introduce a
full socket/core/thread-based device_add interface, setting APIC ID
directly externally won't be necessary anymore (QEMU could calculate it
based on the object topology), but while we don't have the full-featured
core/socket/thread-based interface we can simply use IDs in the range
[0..(max_cpus-1)], that are already used by the "-numa" command-line
option.

-- 
Eduardo