From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel De Graaf Subject: Re: [PATCH v2 2/2] xen/evtchn: optimize XSM ssid field Date: Fri, 21 Mar 2014 10:54:42 -0400 Message-ID: <532C52B2.10206@tycho.nsa.gov> 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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <532C3E93.1080208@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: David Vrabel Cc: Keir Fraser , Jan Beulich , xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org 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