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=-4.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, 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 EE976C433FE for ; Thu, 10 Dec 2020 13:22:08 +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 9352023DE5 for ; Thu, 10 Dec 2020 13:22:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9352023DE5 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.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.49211.86989 (Exim 4.92) (envelope-from ) id 1knLtP-0003iS-0j; Thu, 10 Dec 2020 13:21:59 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 49211.86989; Thu, 10 Dec 2020 13:21:58 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1knLtO-0003iL-Tm; Thu, 10 Dec 2020 13:21:58 +0000 Received: by outflank-mailman (input) for mailman id 49211; Thu, 10 Dec 2020 13:21:57 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1knLtM-0003iG-Vw for xen-devel@lists.xenproject.org; Thu, 10 Dec 2020 13:21:57 +0000 Received: from mail-lf1-x142.google.com (unknown [2a00:1450:4864:20::142]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 532f62c1-e92f-48b9-8877-7f66ac2abf49; Thu, 10 Dec 2020 13:21:55 +0000 (UTC) Received: by mail-lf1-x142.google.com with SMTP id m25so8140879lfc.11 for ; Thu, 10 Dec 2020 05:21:55 -0800 (PST) Received: from [192.168.1.7] ([212.22.223.21]) by smtp.gmail.com with ESMTPSA id y12sm482995lfy.300.2020.12.10.05.21.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Dec 2020 05:21:54 -0800 (PST) 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: 532f62c1-e92f-48b9-8877-7f66ac2abf49 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-transfer-encoding:content-language; bh=NOedhVT3rvhdHXuGqVxU37/Xsir0ZN0vrpQc+/P9xm4=; b=HIAbXnrYpPbBzsPh8PCjdG//01sIMd72psTDBVAXPZmmMaNmcUOEPnqjiqbYwTjacS wcPReKzQb4FyURa6ZWrdPZpKHkdfquk8p1Y8PETCLAr5gXBEpx133DC42Bvq8iCeJqyM J0JWZIPGXWALeZWk4wPFGJE5jeOPdWBqTcsVriMJw0OAObJvNvSkoyMC3CumZl/jojH4 MQ3so+qLcQKrsnyj2RTBJ+cY0NdFoBaYHRnMPQKCjiZd2bteW8ZDtI2/tSitEm+7XsZd YN0F8SqVuS+HHlKzRrVDDolcc/pX2nCRhqLc1kOamc8XCQyhZCHX9o9SPFn1cg7FNs4n fT+w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :content-language; bh=NOedhVT3rvhdHXuGqVxU37/Xsir0ZN0vrpQc+/P9xm4=; b=KC0+1rzC0xIqKF7f0a03Q5e0IlxUqlWFJWYaloJZXn7gwp/Nt0ZgshRKWJrsXAaRw7 ZeH/x5mIu8ZgQue0SCLzYmCsexv9/zE1X0lLKYF/vptm5GL3G2wysi0WIVTnxQ7hF9sQ uqW2oBTLobR1XLKkA++xphhMZyz6N+UILOJD/4roJgFdH9tEE9fCXE6AnZwrQlao+eef I0HaqQjaU40tSulfe6pNBrv2kYvlFiZ20TtblwzESGlrtVKXiYcsLWqCeBSS+shOnfuB fR1yxTVDf/C701BQQIlHoU0kF7h8SD8/393auCt9bWtmln8SaK6NOY1rtUGD8VoNDwBC TGZg== X-Gm-Message-State: AOAM531RlTrw5CGYIQp4cv1dQ3wOf3KLE0yCvhBGt7f+u5F2HqW+QRgB DDptNBqJqT455sFdTijhUhA= X-Google-Smtp-Source: ABdhPJx5/4mH2wr+XsZl8BCvpjmawibTtVzu0mH/VQIu3KglLmkLGls9nL7kqH1M8QbsvZL0GJGHPA== X-Received: by 2002:a19:a57:: with SMTP id 84mr2749675lfk.327.1607606514683; Thu, 10 Dec 2020 05:21:54 -0800 (PST) Subject: Re: [PATCH V3 15/23] xen/arm: Stick around in leave_hypervisor_to_guest until I/O has completed To: Stefano Stabellini , Julien Grall Cc: 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: Oleksandr Message-ID: <88aea7f9-43d2-f67b-b5a8-c0b6204eac58@gmail.com> Date: Thu, 10 Dec 2020 15:21:53 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US On 10.12.20 04:30, Stefano Stabellini wrote: Hi Julien, Stefano > On Wed, 9 Dec 2020, Julien Grall wrote: >> On 09/12/2020 23:35, Stefano Stabellini wrote: >>> On Wed, 9 Dec 2020, Stefano Stabellini wrote: >>>> On Mon, 30 Nov 2020, Oleksandr Tyshchenko wrote: >>>>> From: Oleksandr Tyshchenko >>>>> >>>>> This patch adds proper handling of return value of >>>>> vcpu_ioreq_handle_completion() which involves using a loop >>>>> in leave_hypervisor_to_guest(). >>>>> >>>>> The reason to use an unbounded loop here is the fact that vCPU >>>>> shouldn't continue until an I/O has completed. In Xen case, if an I/O >>>>> never completes then it most likely means that something went horribly >>>>> wrong with the Device Emulator. And it is most likely not safe to >>>>> continue. So letting the vCPU to spin forever if I/O never completes >>>>> is a safer action than letting it continue and leaving the guest in >>>>> unclear state and is the best what we can do for now. >>>>> >>>>> This wouldn't be an issue for Xen as do_softirq() would be called at >>>>> every loop. In case of failure, the guest will crash and the vCPU >>>>> will be unscheduled. >>>> Imagine that we have two guests: one that requires an ioreq server and >>>> one that doesn't. If I am not mistaken this loop could potentially spin >>>> forever on a pcpu, thus preventing any other guest being scheduled, even >>>> if the other guest doesn't need any ioreq servers. >>>> >>>> >>>> My other concern is that we are busy-looping. Could we call something >>>> like wfi() or do_idle() instead? The ioreq server event notification of >>>> completion should wake us up? >>>> >>>> Following this line of thinking, I am wondering if instead of the >>>> busy-loop we should call vcpu_block_unless_event_pending(current) in >>>> try_handle_mmio if IO_RETRY. Then when the emulation is done, QEMU (or >>>> equivalent) calls xenevtchn_notify which ends up waking up the domU >>>> vcpu. Would that work? >>> I read now Julien's reply: we are already doing something similar to >>> what I suggested with the following call chain: >>> >>> check_for_vcpu_work -> vcpu_ioreq_handle_completion -> wait_for_io -> >>> wait_on_xen_event_channel >>> >>> So the busy-loop here is only a safety-belt in cause of a spurious >>> wake-up, in which case we are going to call again check_for_vcpu_work, >>> potentially causing a guest reschedule. >>> >>> Then, this is fine and addresses both my concerns. Maybe let's add a note >>> in the commit message about it. >> Damm, I hit the "sent" button just a second before seen your reply. :/ Oh >> well. I suggested the same because I have seen the same question multiple >> time. I will update commit description and probably the comment in code. -- Regards, Oleksandr Tyshchenko