From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AD508CA9EB5 for ; Mon, 4 Nov 2019 15:09:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8143F204EC for ; Mon, 4 Nov 2019 15:09:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729118AbfKDPJl (ORCPT ); Mon, 4 Nov 2019 10:09:41 -0500 Received: from mx2.suse.de ([195.135.220.15]:37118 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727861AbfKDPJl (ORCPT ); Mon, 4 Nov 2019 10:09:41 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A700BADD5; Mon, 4 Nov 2019 15:09:39 +0000 (UTC) Subject: Re: [Xen-devel] [PATCH] xen/events: remove event handling recursion detection To: Jan Beulich Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Stefano Stabellini , Boris Ostrovsky References: <20191104135812.2314-1-jgross@suse.com> <40cba9d9-24b0-3141-4ba8-02e03049f1bf@suse.com> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Message-ID: Date: Mon, 4 Nov 2019 16:09:38 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <40cba9d9-24b0-3141-4ba8-02e03049f1bf@suse.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04.11.19 15:35, Jan Beulich wrote: > On 04.11.2019 14:58, Juergen Gross wrote: >> __xen_evtchn_do_upcall() contains guards against being called >> recursively. This mechanism was introduced in the early pvops times >> (kernel 2.6.26) when there were still Xen versions around not honoring >> disabled interrupts for sending events to pv guests. >> >> This was changed in Xen 3.0, which is much older than any Xen version >> supported by the kernel, so the recursion detection can be removed. > > Would you mind pointing out which exact change(s) this was(were)? Linux kernel: 229664bee6126e01f8662976a5fe2e79813b77c8 Xen: d8263e8dbaf5ef1445bee0662143a0fcb6d43466 > It had always been my understanding that the recursion detection > was mainly to guard against drivers re-enabling interrupts > transiently in their handlers (which in turn may no longer be an > issue in modern Linux kernels). This would have been doable with a simple bool. The more complex xchg based logic was IMO for recursion detection at any point. Juergen