Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757529AbeAIFf7 (ORCPT + 1 other); Tue, 9 Jan 2018 00:35:59 -0500 Received: from mx2.suse.de ([195.135.220.15]:44455 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755192AbeAIFf6 (ORCPT ); Tue, 9 Jan 2018 00:35:58 -0500 Subject: Re: [PATCH v2] x86: xen: remove the use of VLAIS To: Ingo Molnar Cc: Peter Zijlstra , Nick Desaulniers , ghackmann@google.com, mka@google.com, kees@google.com, srhines@google.com, Boris Ostrovsky , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org References: <1515274788-24548-1-git-send-email-nick.desaulniers@gmail.com> <20180108161001.GM32035@hirez.programming.kicks-ass.net> <892cb5f2-367b-cfbd-58b5-3d8711c3ed66@suse.com> <20180108184939.6rckuyy77qt3ureg@gmail.com> From: Juergen Gross Message-ID: Date: Tue, 9 Jan 2018 06:35:54 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: <20180108184939.6rckuyy77qt3ureg@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On 08/01/18 19:49, Ingo Molnar wrote: > > * Juergen Gross wrote: > >> On 08/01/18 17:10, Peter Zijlstra wrote: >>> On Sat, Jan 06, 2018 at 01:39:48PM -0800, Nick Desaulniers wrote: >>>> Variable Length Arrays In Structs (VLAIS) is not supported by Clang, and >>>> frowned upon by others. >>>> >>>> https://lkml.org/lkml/2013/9/23/500 >>>> >>>> Here, the VLAIS was used because the size of the bitmap returned from >>>> xen_mc_entry() depended on possibly (based on kernel configuration) >>>> runtime sized data. Rather than declaring args as a VLAIS then calling >>>> sizeof on *args, we calculate the appropriate sizeof args manually. >>>> Further, we can get rid of the #ifdef's and rely on num_possible_cpus() >>>> (thanks to a helpful checkpatch warning from an earlier version of this >>>> patch). >>>> >>>> Suggested-by: Juergen Gross >>>> Signed-off-by: Nick Desaulniers >>>> --- >>>> Changes in v2: >>>> * Change mask to us DECLARE_BITMAP instead of pointer, as suggested. >>>> * Update commit message to remove mention of pointer. >>>> * Update sizeof calculation to work with array rather than pointer. >>>> >>>> arch/x86/xen/mmu_pv.c | 8 +++----- >>>> 1 file changed, 3 insertions(+), 5 deletions(-) >>>> >>>> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c >>>> index 4d62c07..d850762 100644 >>>> --- a/arch/x86/xen/mmu_pv.c >>>> +++ b/arch/x86/xen/mmu_pv.c >>>> @@ -1325,20 +1325,18 @@ static void xen_flush_tlb_others(const struct cpumask *cpus, >>>> { >>>> struct { >>>> struct mmuext_op op; >>>> -#ifdef CONFIG_SMP >>>> - DECLARE_BITMAP(mask, num_processors); >>>> -#else >>>> DECLARE_BITMAP(mask, NR_CPUS); >>>> -#endif >>>> } *args; >>> >>> Why is it OK for Xen to place this bitmap on-stack in the first place? >>> That NR_CPUS thing can be fairly huge. >> >> This only a pointer to the bitmap. > > What's the maximum NR_CPUs for configs that can run this code, times 8? Why does this matter? args is a pointer only, so it occupies 8 bytes of the stack. The structure is only for type correctness. Juergen