From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juergen Gross Subject: Re: [PATCH 5/5] xen: clean up VPF flags macros Date: Mon, 28 Sep 2015 10:44:24 +0200 Message-ID: <5608FDE8.20001@suse.com> References: <1443182072-15321-1-git-send-email-jgross@suse.com> <1443182072-15321-6-git-send-email-jgross@suse.com> <5605878D02000078000A5BA1@suse.com> <5608CEC1.1080207@suse.com> <5608F8AE02000078000A5F6E@suse.com> <5608EC5B.3060106@suse.com> <56090DDF02000078000A6042@suse.com> <5608F71D.6040608@suse.com> <560917CE02000078000A6085@suse.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <560917CE02000078000A6085@suse.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: Jan Beulich Cc: tim@xen.org, wei.liu2@citrix.com, Ian.Campbell@citrix.com, stefano.stabellini@eu.citrix.com, andrew.cooper3@citrix.com, ian.jackson@eu.citrix.com, xen-devel@lists.xen.org, david.vrabel@citrix.com, keir@xen.org List-Id: xen-devel@lists.xenproject.org On 09/28/2015 10:34 AM, Jan Beulich wrote: >>>> On 28.09.15 at 10:15, wrote: >> On 09/28/2015 09:52 AM, Jan Beulich wrote: >>>>>> On 28.09.15 at 09:29, wrote: >>>> On 09/28/2015 08:22 AM, Jan Beulich wrote: >>>>>>>> On 28.09.15 at 07:23, wrote: >>>>>> On 09/25/2015 05:42 PM, Jan Beulich wrote: >>>>>>>>>> On 25.09.15 at 13:54, wrote: >>>>>>>> --- a/xen/common/domctl.c >>>>>>>> +++ b/xen/common/domctl.c >>>>>>>> @@ -172,7 +172,7 @@ void getdomaininfo(struct domain *d, struct >>>>>> xen_domctl_getdomaininfo *info) >>>>>>>> info->max_vcpu_id = v->vcpu_id; >>>>>>>> if ( !test_bit(_VPF_down, &v->pause_flags) ) >>>>>>>> { >>>>>>>> - if ( !(v->pause_flags & VPF_blocked) ) >>>>>>>> + if ( !test_bit(_VPF_blocked, &v->pause_flags) ) >>>>>>> >>>>>>> test_bit() is quite a bit more complex an operation than a simple &, >>>>>>> and with (on x86) even constant_test_bit() involving a cast to >>>>>>> a pointer to volatile I'm afraid we can't even hope that compilers >>>>>>> would produce identical code for both in cases like this one (as that >>>>>>> casts limits freedom of the compiler). IOW I'd rather see other >>>>>>> test_bit(_VPF_...) uses converted the inverse way (which as a nice >>>>>>> but minor side effect would yield slightly smaller source code). >>>>>> >>>>>> What about introducing __test_bit() being a variant which can be >>>>>> reordered by omitting the volatile modifier? I think this would have >>>>>> the same effect. >>>>> >>>>> I'm not convinced it always would - the inline function is still more >>>>> complex than the plain operation. >>>> >>>> Depends on the way it is done. What about: >>>> >>>> #define __test_bit(nr, addr) ({ \ >>>> if ( bitop_bad_size(addr) ) __bitop_bad_size(); \ >>>> (__builtin_constant_p(nr) ? \ >>>> !!(*(addr) & ((typeof)(*(addr))1 << (nr))) : \ >>>> __variable_test_bit((nr),(addr))); \ >>>> }) >>> >>> But that's not correct - addr may point to wider than a single entry >>> array, irrespective of whether nr is a compile time constant. >>> >>>> It would even be possible to drop the test for bitop_bad_size(addr) in >>>> the constant case. >>> >>> In which case 1 << nr may reference a bit beyond the type >>> of *addr. >> >> Hmm, yes, you are right, of course. >> >> It could be fixed, however. >> >> The question is: does it make sense to follow this path any longer, >> or would you reject it even in case of correctness? I wouldn't mind >> either way, I just don't want to waste time (mine and yours). > > I continue to think that the better route would be to get rid of the > unnecessary test_bit() uses in favor of the shorter and less > restrictive (to the compiler) & operation. Okay, I'll follow that route then. Juergen