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 99866C63777 for ; Thu, 3 Dec 2020 09:46:50 +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 21AD220C56 for ; Thu, 3 Dec 2020 09:46:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 21AD220C56 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com 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.43275.77824 (Exim 4.92) (envelope-from ) id 1kklC9-0004WV-Kp; Thu, 03 Dec 2020 09:46:37 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 43275.77824; Thu, 03 Dec 2020 09:46:37 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kklC9-0004WO-He; Thu, 03 Dec 2020 09:46:37 +0000 Received: by outflank-mailman (input) for mailman id 43275; Thu, 03 Dec 2020 09:46:36 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kklC8-0004WJ-R6 for xen-devel@lists.xenproject.org; Thu, 03 Dec 2020 09:46:36 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 5c04f2bc-b956-40c1-a54c-27a32343a6fa; Thu, 03 Dec 2020 09:46:35 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id E26F8ACBA; Thu, 3 Dec 2020 09:46:34 +0000 (UTC) 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" X-Inumbo-ID: 5c04f2bc-b956-40c1-a54c-27a32343a6fa X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1606988795; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QeXzYmmdPu0yiACCqNFWW9iWhMTg03koey4W+Yzot1g=; b=pSQOWjddrYPJkGjNIT71GF/j2seaEqNTHA59ySjkrwJhNWada7enK18XLqpSUOQ35aVHe1 zGywIoIGselbKFSv61tnfkwQ0Aze25IDP8pHU4Pv/dIK7lQXboqpkJA1Loszda9nZVj6QX aRm2WDiSkN8rsKwzZXmwOtW7/W4M3Dg= Subject: Re: [PATCH v3 1/5] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq() To: Julien Grall 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: Jan Beulich Message-ID: Date: Thu, 3 Dec 2020 10:46:34 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <70170293-a9a7-282a-dde6-7ed73fc2da48@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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. And this change eliminates 50% of them, with the added benefit of making the paths more lightweight. Jan