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 15:16:35 +0000 Message-ID: <532C57D3.60509@citrix.com> References: <1395329382-11310-1-git-send-email-dgdegra@tycho.nsa.gov> <1395329382-11310-2-git-send-email-dgdegra@tycho.nsa.gov> <532C3E93.1080208@citrix.com> <532C52B2.10206@tycho.nsa.gov> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <532C52B2.10206@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 21/03/14 14:54, Daniel De Graaf wrote: > 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). Oh, now I remember why I didn't do this before. Sorry for wasting your time. David