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=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,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 0384CC4361B for ; Wed, 9 Dec 2020 09:53:44 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9DF8523407 for ; Wed, 9 Dec 2020 09:53:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9DF8523407 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=xen.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.48076.85045 (Exim 4.92) (envelope-from ) id 1kmwA5-0003iS-Ff; Wed, 09 Dec 2020 09:53:29 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 48076.85045; Wed, 09 Dec 2020 09:53:29 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmwA5-0003iL-Cd; Wed, 09 Dec 2020 09:53:29 +0000 Received: by outflank-mailman (input) for mailman id 48076; Wed, 09 Dec 2020 09:53:28 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmwA3-0003iG-VM for xen-devel@lists.xenproject.org; Wed, 09 Dec 2020 09:53:28 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmwA2-000424-W3; Wed, 09 Dec 2020 09:53:26 +0000 Received: from [54.239.6.186] (helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1kmwA2-0004aI-Nk; Wed, 09 Dec 2020 09:53:26 +0000 X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject; bh=CnzN0qywqES9bUxXUiV2DG77UGN62F8l63o8oWlkDBA=; b=XKxOQiFCukGOeXckYI/KSJPyil JV/PjG7IjycEtANuFobDgY+TXmMDVwc23HoHqTds9wuGJwyRb6067QSUuPgWNgPdI3+YfYUNm//yx a+OVQT7aHwtNO87l0ebx4pIirraYkKLyL+leLYo/L8ub9o9tg2BGw8OzbmpTBzv1DvjU=; Subject: Re: [PATCH v3 1/5] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq() To: Jan Beulich Cc: Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , "xen-devel@lists.xenproject.org" References: <9d7a052a-6222-80ff-cbf1-612d4ca50c2a@suse.com> <70170293-a9a7-282a-dde6-7ed73fc2da48@xen.org> From: Julien Grall Message-ID: Date: Wed, 9 Dec 2020 09:53:24 +0000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Hi Jan, On 03/12/2020 09:46, Jan Beulich wrote: > On 02.12.2020 20:03, Julien Grall wrote: >> On 23/11/2020 13:28, Jan Beulich wrote: >>> The per-vCPU virq_lock, which is being held anyway, together with there >>> not being any call to evtchn_port_set_pending() when v->virq_to_evtchn[] >>> is zero, provide sufficient guarantees. >> >> I agree that the per-vCPU virq_lock is going to be sufficient, however >> dropping the lock also means the event channel locking is more complex >> to understand (the long comment that was added proves it). >> >> In fact, the locking in the event channel code was already proven to be >> quite fragile, therefore I think this patch is not worth the risk. > > I agree this is a very reasonable position to take. I probably > would even have remained silent if in the meantime the > spin_lock()s there hadn't changed to read_trylock()s. I really > think we want to limit this unusual locking model to where we > strictly need it. While I appreciate that the current locking is unusual, we should follow the same model everywhere rather than having a dozen of way to lock the same structure. The rationale is quite simple, if you have one way to lock a structure, then there are less chance to screw up. The only reason I would be willing to diverge from statement is if the performance are significantly improved. Cheers, -- Julien Grall