From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: [PATCH v2 2/2] xen/evtchn: optimize XSM ssid field Date: Fri, 21 Mar 2014 13:28:51 +0000 Message-ID: <532C3E93.1080208@citrix.com> References: <1395329382-11310-1-git-send-email-dgdegra@tycho.nsa.gov> <1395329382-11310-2-git-send-email-dgdegra@tycho.nsa.gov> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1395329382-11310-2-git-send-email-dgdegra@tycho.nsa.gov> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Daniel De Graaf Cc: Keir Fraser , Jan Beulich , xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org 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