Re: [RFC PATCH 3/7] xen: psr: reserve an RMID for each core

[George Dunlap <george.dunlap@eu.citrix.com>]

On 04/04/2015 03:14 AM, Dario Faggioli wrote:
> This allows for a new item to be passed as part of the psr=
> boot option: "percpu_cmt". If that is specified, Xen tries,
> at boot time, to associate an RMID to each core.
> 
> XXX This all looks rather straightforward, if it weren't
>     for the fact that it is, apparently, more common than
>     I though to run out of RMID. For example, on a dev box
>     we have in Cambridge, there are 144 pCPUs and only 71
>     RMIDs.

Is that because you have 2 sockets?

There's no need to keep RMIDs unique across sockets, is there?  E.g.,
socket 0 cpu 0 and socket 1 cpu 0 can have the same RMID, because cache
and the MSRs are per-socket.

If we're doing things on a per-domain basis, having the same RMID
allocated for each socket sort of makes sense; but even then, if you
know a domain is only going to run on a given socket, there's no reason
in theory we couldn't use same RMID for a different domain on the other
socket (assuming it was only going to run on the other socket).

One advantage of doing things of a per-vcpu level is that you wouldn't
have to worry about inter-socket RMID issues.

 -George