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 B5494C433FE for ; Thu, 10 Dec 2020 13:17:57 +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 6B11323DE8 for ; Thu, 10 Dec 2020 13:17:57 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6B11323DE8 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.49205.86977 (Exim 4.92) (envelope-from ) id 1knLpH-0002kY-ER; Thu, 10 Dec 2020 13:17:43 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 49205.86977; Thu, 10 Dec 2020 13:17:43 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1knLpH-0002kR-BT; Thu, 10 Dec 2020 13:17:43 +0000 Received: by outflank-mailman (input) for mailman id 49205; Thu, 10 Dec 2020 13:17:42 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1knLpF-0002kM-WC for xen-devel@lists.xenproject.org; Thu, 10 Dec 2020 13:17:42 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1knLpE-0007La-HA; Thu, 10 Dec 2020 13:17:40 +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 1knLpE-00086w-7w; Thu, 10 Dec 2020 13:17:40 +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=fCgGDOSwt7p/h6DxE1W0vBRr6YFicucTeTT1d4kB45k=; b=W8WLuYcy5i+A/goZtaE3F952eL sw8heASU8yYCa8EGKvOmGGb7DOrYOAO/I6F8uOMT0hmdD/Tj+4MhfxaJuQYMEYuXBmSv3zbU00ZtP +kY5gqP5BaeXzjHovJi4h2l1TuP9xD2IQnvmAcuKBhUba0+DQIDTCSoAX8nD9R5e3Sp4=; Subject: Re: [PATCH V3 15/23] xen/arm: Stick around in leave_hypervisor_to_guest until I/O has completed To: Stefano Stabellini Cc: Oleksandr Tyshchenko , xen-devel@lists.xenproject.org, Oleksandr Tyshchenko , Volodymyr Babchuk , Julien Grall References: <1606732298-22107-1-git-send-email-olekstysh@gmail.com> <1606732298-22107-16-git-send-email-olekstysh@gmail.com> <52799b99-6405-03f4-2a46-3a0a4aac597f@xen.org> From: Julien Grall Message-ID: <6053ad08-95ce-fb31-122d-450df21a20f7@xen.org> Date: Thu, 10 Dec 2020 13:17:38 +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 Stefano, On 10/12/2020 02:30, Stefano Stabellini wrote: >>> I am also wondering if there is any benefit in calling wait_for_io() >>> earlier, maybe from try_handle_mmio if IO_RETRY? >> >> wait_for_io() may end up to deschedule the vCPU. I would like to avoid this to >> happen in the middle of the I/O emulation because we need to happen it without >> lock held at all. >> >> I don't think there are locks involved today, but the deeper in the call stack >> the scheduling happens, the more chance we may screw up in the future. >> >> However... >> >>> leave_hypervisor_to_guest is very late for that. >> >> ... I am not sure what's the problem with that. The IOREQ will be notified of >> the pending I/O as soon as try_handle_mmio() put the I/O in the shared page. >> >> If the IOREQ server is running on a different pCPU, then it might be possible >> that the I/O has completed before reached leave_hypervisor_to_guest(). In this >> case, we would not have to wait for the I/O. > > Yeah, I was thinking about that too. Actually it could be faster > this way we end up being lucky. > > The reason for moving it earlier would be that by the time > leave_hypervisor_to_guest is called "Xen" has already decided to > continue running this particular vcpu. If we called wait_for_io() > earlier, we would give important information to the scheduler before any > decision is made. I don't understand this. Xen preemption is voluntary, so the scheduler is not going to run unless requested. wait_for_io() is a preemption point. So if you call it, then vCPU may get descheduled at that point. Why would we want to do this? What's our benefits here? Cheers, -- Julien Grall