Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755973AbeAHStp (ORCPT + 1 other); Mon, 8 Jan 2018 13:49:45 -0500 Received: from mail-wm0-f66.google.com ([74.125.82.66]:42022 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755501AbeAHStn (ORCPT ); Mon, 8 Jan 2018 13:49:43 -0500 X-Google-Smtp-Source: ACJfBosr2DSm65XdLwXkHcMy/9mpRmtBxzcVuVieRpCkEUitjlR3CPEGgPEevCjxXk9+K43aE5N4ww== Date: Mon, 8 Jan 2018 19:49:39 +0100 From: Ingo Molnar To: Juergen Gross 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 Subject: Re: [PATCH v2] x86: xen: remove the use of VLAIS Message-ID: <20180108184939.6rckuyy77qt3ureg@gmail.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <892cb5f2-367b-cfbd-58b5-3d8711c3ed66@suse.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: * 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? Thanks, Ingo