Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934983AbeAHQ0q (ORCPT + 1 other); Mon, 8 Jan 2018 11:26:46 -0500 Received: from mx2.suse.de ([195.135.220.15]:56091 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932236AbeAHQ0o (ORCPT ); Mon, 8 Jan 2018 11:26:44 -0500 Subject: Re: [PATCH v2] x86: xen: remove the use of VLAIS To: Peter Zijlstra , Nick Desaulniers Cc: 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> From: Juergen Gross Message-ID: <892cb5f2-367b-cfbd-58b5-3d8711c3ed66@suse.com> Date: Mon, 8 Jan 2018 17:26:39 +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: <20180108161001.GM32035@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: de-DE 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 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. Juergen