Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933475AbeAHQgT (ORCPT + 1 other); Mon, 8 Jan 2018 11:36:19 -0500 Received: from userp2130.oracle.com ([156.151.31.86]:43232 "EHLO userp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756153AbeAHQgQ (ORCPT ); Mon, 8 Jan 2018 11:36:16 -0500 Subject: Re: [Xen-devel] [PATCH v2] x86: xen: remove the use of VLAIS To: Juergen Gross , Peter Zijlstra , Nick Desaulniers Cc: x86@kernel.org, "H. Peter Anvin" , linux-kernel@vger.kernel.org, ghackmann@google.com, mka@google.com, Ingo Molnar , srhines@google.com, kees@google.com, xen-devel@lists.xenproject.org, Thomas Gleixner References: <1515274788-24548-1-git-send-email-nick.desaulniers@gmail.com> <20180108161001.GM32035@hirez.programming.kicks-ass.net> <0745372f-93af-3b70-e82d-db75af32742a@oracle.com> <62287687-99c2-b306-d1fc-7dc87fd38eb6@suse.com> From: Boris Ostrovsky Message-ID: Date: Mon, 8 Jan 2018 11:35:41 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <62287687-99c2-b306-d1fc-7dc87fd38eb6@suse.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=8768 signatures=668652 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1711220000 definitions=main-1801080235 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On 01/08/2018 11:28 AM, Juergen Gross wrote: > On 08/01/18 17:20, Boris Ostrovsky wrote: >> On 01/08/2018 11:10 AM, 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. >> Err... right. Now it's even worse than it was before, when the array was >> potentially smaller. I'll drop this. > No, its only the pointer to the struct, not the struct itself. > It's the full array, isn't it? #define DECLARE_BITMAP(name,bits) \ unsigned long name[BITS_TO_LONGS(bits)] OK, it *is* a pointer. Sigh... -boris