From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH 1/2] xen, libxc: Introduced XEN_DOMCTL_emulate_each_rep Date: Thu, 17 Sep 2015 14:37:59 +0100 Message-ID: <55FAC237.8070702@citrix.com> References: <1442308784-10419-1-git-send-email-rcojocaru@bitdefender.com> <1442308784-10419-2-git-send-email-rcojocaru@bitdefender.com> <55FAB936.2090202@citrix.com> <55FABE0C.8050902@bitdefender.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55FABE0C.8050902@bitdefender.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: Razvan Cojocaru , xen-devel@lists.xen.org Cc: tamas@tklengyel.com, keir@xen.org, ian.campbell@citrix.com, stefano.stabellini@eu.citrix.com, george.dunlap@eu.citrix.com, ian.jackson@eu.citrix.com, jbeulich@suse.com, wei.liu2@citrix.com List-Id: xen-devel@lists.xenproject.org On 17/09/15 14:20, Razvan Cojocaru wrote: > On 09/17/2015 03:59 PM, Andrew Cooper wrote: >> On 15/09/15 10:19, Razvan Cojocaru wrote: >>> Previously, if vm_event emulation support was enabled, then REP >>> optimizations were disabled when emulating REP-compatible >>> instructions. This patch allows fine-tuning of this behaviour by >>> providing a dedicated libxc helper function. >>> >>> Signed-off-by: Razvan Cojocaru >> This disables all rep optimisations by default, so on its own is >> inappropriate. > REP optimizations are enabled by default. Emulate_each_rep is initially > set to 0, when struct hvm_domain is being initialized, which means that > REP optimizations are enabled. I've tested this and it does work, am I > missing something? Oops - you are completely correct. I got the logic reversed in my head. Sorry for the noise. > >> I am also not sure that an individual domctl subop is appropriate. Its >> purpose is to undo a performance hit caused by introspection, so should >> live as an introspection subop IMO. > Do you mean xc_monitor_emulate_each_rep() instead of > xc_domain_emulate_each_rep()? > > I've placed this in its own domctl subop because it's not introspection > (or vm_event) specific. The change in > xen/arch/x86/hvm/emulate.c enables / disables REP emulation > optimizations regardless of whether there's a vm_event client or not. I > thought this might come handy for somebody else too. I can't think of a rational reason for anyone to disable rep optimisations for the sake of it. I am concerned about introducing options with which people can needlessly shoot themselves in the foot. On the other hand, there are already enough of those. ~Andrew