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=-7.2 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,URIBL_BLOCKED,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 1FA52C4361B for ; Wed, 9 Dec 2020 23:48:10 +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 B9B3D23B45 for ; Wed, 9 Dec 2020 23:48:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B9B3D23B45 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.48878.86477 (Exim 4.92) (envelope-from ) id 1kn9BZ-0003BB-CW; Wed, 09 Dec 2020 23:47:53 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 48878.86477; Wed, 09 Dec 2020 23:47:53 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kn9BZ-0003B4-8p; Wed, 09 Dec 2020 23:47:53 +0000 Received: by outflank-mailman (input) for mailman id 48878; Wed, 09 Dec 2020 23:47:51 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kn9BX-0003Az-9L for xen-devel@lists.xenproject.org; Wed, 09 Dec 2020 23:47:51 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kn9BV-0005VF-UI; Wed, 09 Dec 2020 23:47:49 +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 1kn9BV-0003Mp-Lu; Wed, 09 Dec 2020 23:47:49 +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=UONJBA/KX+vQqohI3/fhkl5eeQUbAO/CODqEwYJ967o=; b=oOfhfeHephQ8rgxIDGdlY10Ijp DxxN5YwGMFpJkiYKsdbp+icgQEO3TJSyKo7K1tasOf+ikcbWK/fZp08Lc7WrW6oAqC8V+YLZsM7TD EvB+iJnKRtT0U3jf+ygLz5jS4FRB1X8aLzvYeawQrtZQLzr7EE/I4zkCUUO2b8/LdIXE=; 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> From: Julien Grall Message-ID: <52799b99-6405-03f4-2a46-3a0a4aac597f@xen.org> Date: Wed, 9 Dec 2020 23:47:47 +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 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 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. Cheers, -- Julien Grall