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

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

On 03/21/2014 09:28 AM, David Vrabel wrote:
> 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

Making this work correctly requires adding__packed to the interdomain
structure (otherwise, the padding just remains at the end); with this,
the structure is 24 bytes without XSM, 28 with FLASK only, and 32 with
XSM_NEED_GENERIC_EVTCHN_SSID (all with one byte of padding remaining).

Using __packed on the structure containing only 8-byte field makes
alignof(struct evtchn) == 4.  I believe this needs to be fixed by adding
an __attribute__((__aligned__(alignof(void*)))) to the structure; should
this be wrapped in some type of macro similar to __packed?

-- 
Daniel De Graaf
National Security Agency