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=-1.2 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS,UNPARSEABLE_RELAY autolearn=ham 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 93BD8C43381 for ; Wed, 20 Feb 2019 17:07:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5BDCB218C3 for ; Wed, 20 Feb 2019 17:07:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="oIoFKHX8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726224AbfBTRHq (ORCPT ); Wed, 20 Feb 2019 12:07:46 -0500 Received: from userp2120.oracle.com ([156.151.31.85]:39740 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725796AbfBTRHq (ORCPT ); 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Wed, 20 Feb 2019 17:07:35 +0000 Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserv0022.oracle.com (8.14.4/8.14.4) with ESMTP id x1KH7XNs005707 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 20 Feb 2019 17:07:34 GMT Received: from abhmp0007.oracle.com (abhmp0007.oracle.com [141.146.116.13]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x1KH7WXa006513; Wed, 20 Feb 2019 17:07:32 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 20 Feb 2019 09:07:32 -0800 Subject: Re: xen/evtchn and forced threaded irq To: Julien Grall Cc: Juergen Gross , Stefano Stabellini , xen-devel , "linux-kernel@vger.kernel.org" , Dave P Martin References: <5e256d9a-572c-e01e-7706-407f99245b00@arm.com> <20190220000209.GA4091@localhost.localdomain> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9173 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=964 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1902200120 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/20/19 9:15 AM, Julien Grall wrote: > Hi Boris, > > Thank you for your answer. > > On 20/02/2019 00:02, Boris Ostrovsky wrote: >> On Tue, Feb 19, 2019 at 05:31:10PM +0000, Julien Grall wrote: >>> Hi all, >>> >>> I have been looking at using Linux RT in Dom0. Once the guest is >>> started, >>> the console is ending to have a lot of warning (see trace below). >>> >>> After some investigation, this is because the irq handler will now >>> be threaded. >>> I can reproduce the same error with the vanilla Linux when passing >>> the option >>> 'threadirqs' on the command line (the trace below is from 5.0.0-rc7 >>> that has >>> not RT support). >>> >>> FWIW, the interrupt for port 6 is used to for the guest to >>> communicate with >>> xenstore. >>> >>>  From my understanding, this is happening because the interrupt >>> handler is now >>> run in a thread. So we can have the following happening. >>> >>>     Interrupt context            |     Interrupt thread >>>                                  | >>>     receive interrupt port 6     | >>>     clear the evtchn port        | >>>     set IRQF_RUNTHREAD            | >>>     kick interrupt thread        | >>>                                  |    clear IRQF_RUNTHREAD >>>                                  |    call evtchn_interrupt >>>     receive interrupt port 6     | >>>     clear the evtchn port        | >>>     set IRQF_RUNTHREAD           | >>>     kick interrupt thread        | >>>                                  |    disable interrupt port 6 >>>                                  |    evtchn->enabled = false >>>                                  |    [....] >>>                                  | >>>                                  |    *** Handling the second >>> interrupt *** >>>                                  |    clear IRQF_RUNTHREAD >>>                                  |    call evtchn_interrupt >>>                                  |    WARN(...) >>> >>> I am not entirely sure how to fix this. I have two solutions in mind: >>> >>> 1) Prevent the interrupt handler to be threaded. We would also need to >>> switch from spin_lock to raw_spin_lock as the former may sleep on >>> RT-Linux. >>> >>> 2) Remove the warning >> >> I think access to evtchn->enabled is racy so (with or without the >> warning) we can't use it reliably. > > Thinking about it, it would not be the only issue. The ring is sized > to contain only one instance of the same event. So if you receive > twice the event, you may overflow the ring. Hm... That's another argument in favor of "unthreading" the handler. > >> >> Another alternative could be to queue the irq if !evtchn->enabled and >> handle it in evtchn_write() (which is where irq is supposed to be >> re-enabled). > What do you mean by queue? Is it queueing in the ring? No, I was thinking about having a new structure for deferred interrupts. -boris > If so, wouldn't it be racy as described above?