From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com [148.163.158.5]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3xYN7B6MC4zDrLN for ; Fri, 18 Aug 2017 09:42:46 +1000 (AEST) Received: from pps.filterd (m0098416.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v7HNd4Qh066679 for ; Thu, 17 Aug 2017 19:42:44 -0400 Received: from e15.ny.us.ibm.com (e15.ny.us.ibm.com [129.33.205.205]) by mx0b-001b2d01.pphosted.com with ESMTP id 2cdmac1wjd-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 17 Aug 2017 19:42:44 -0400 Received: from localhost by e15.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 17 Aug 2017 19:42:43 -0400 Date: Thu, 17 Aug 2017 16:42:31 -0700 From: Ram Pai To: Michael Ellerman Cc: Thiago Jung Bauermann , linux-arch@vger.kernel.org, corbet@lwn.net, arnd@arndb.de, linux-doc@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, mhocko@kernel.org, linux-mm@kvack.org, dave.hansen@intel.com, mingo@redhat.com, paulus@samba.org, aneesh.kumar@linux.vnet.ibm.com, linux-kselftest@vger.kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, khandual@linux.vnet.ibm.com Subject: Re: [RFC v6 21/62] powerpc: introduce execute-only pkey Reply-To: Ram Pai References: <1500177424-13695-1-git-send-email-linuxram@us.ibm.com> <1500177424-13695-22-git-send-email-linuxram@us.ibm.com> <87shhgdx5i.fsf@linux.vnet.ibm.com> <87d18fu6o1.fsf@concordia.ellerman.id.au> <87d18fw9it.fsf@linux.vnet.ibm.com> <871sous3xd.fsf@concordia.ellerman.id.au> <20170817233555.GC5427@ram.oc3035372033.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20170817233555.GC5427@ram.oc3035372033.ibm.com> Message-Id: <20170817234231.GA5445@ram.oc3035372033.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Aug 17, 2017 at 04:35:55PM -0700, Ram Pai wrote: > On Wed, Aug 02, 2017 at 07:40:46PM +1000, Michael Ellerman wrote: > > Thiago Jung Bauermann writes: > > > > > Michael Ellerman writes: > > > > > >> Thiago Jung Bauermann writes: > > >>> Ram Pai writes: > > >> ... > > >>>> + > > >>>> + /* We got one, store it and use it from here on out */ > > >>>> + if (need_to_set_mm_pkey) > > >>>> + mm->context.execute_only_pkey = execute_only_pkey; > > >>>> + return execute_only_pkey; > > >>>> +} > > >>> > > >>> If you follow the code flow in __execute_only_pkey, the AMR and UAMOR > > >>> are read 3 times in total, and AMR is written twice. IAMR is read and > > >>> written twice. Since they are SPRs and access to them is slow (or isn't > > >>> it?), > > >> > > >> SPRs read/writes are slow, but they're not *that* slow in comparison to > > >> a system call (which I think is where this code is being called?). > > > > > > Yes, this code runs on mprotect and mmap syscalls if the memory is > > > requested to have execute but not read nor write permissions. > > > > Yep. That's not in the fast path for key usage, ie. the fast path is > > userspace changing the AMR itself, and the overhead of a syscall is > > already hundreds of cycles. > > > > >> So we should try to avoid too many SPR read/writes, but at the same time > > >> we can accept more than the minimum if it makes the code much easier to > > >> follow. > > > > > > Ok. Ram had asked me to suggest a way to optimize the SPR reads and > > > writes and I came up with the patch below. Do you think it's worth it? > > > > At a glance no I don't think it is. Sorry you spent that much time on it. > > > > I think we can probably reduce the number of SPR accesses without > > needing to go to that level of complexity. > > > > But don't throw the patch away, I may eat my words once I have the full > > series applied and am looking at it hard - at the moment I'm just > > reviewing the patches piecemeal as I get time. > Thiago's patch does save some cycles. I dont feel like throwing his work. I agree, It should be considered after applying all the patches. RP -- Ram Pai