Re: [PATCH v2 2/2] xen/evtchn: optimize XSM ssid field

[David Vrabel <david.vrabel@citrix.com>]

On 20/03/14 15:29, Daniel De Graaf wrote:
> When FLASK is the only enabled implementation of the XSM hooks in Xen,
> some of the abstractions required to handle multiple XSM providers are
> redundant and only produce unneeded overhead.  This patch reduces the
> memory overhead of enabling XSM on event channels by replacing the
> untyped ssid pointer from struct evtchn with a union containing the
> contents of the structure.  This avoids an additional heap allocation
> for every event channel, and on 64-bit systems, reduces the size of
> struct evtchn by 4 bytes.

Without XSM the 64-bit structure is 29 bytes (so 32 including the
trailing padding).

Adding a 4 byte word or a 8 byte word both results in a 40 byte
structure which halves the number of evtchns per page (since each page
contains a power of two structs).

I think you could swap the order of the fields in u.interdomain to get
better packing with the 4 byte flask_sid and end up with a 32 byte
struct evtchn (there's 6 bytes of padding between remote_port and
remote_dom).

You may want to check the EVTCHNS_PER_BUCKET value before/after.

David