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 D5F22C4361B for ; Mon, 7 Dec 2020 17:22:52 +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 888922388D for ; Mon, 7 Dec 2020 17:22:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 888922388D 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.46864.83036 (Exim 4.92) (envelope-from ) id 1kmKDg-0008Kp-BF; Mon, 07 Dec 2020 17:22:40 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 46864.83036; Mon, 07 Dec 2020 17:22:40 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmKDg-0008Ki-8D; Mon, 07 Dec 2020 17:22:40 +0000 Received: by outflank-mailman (input) for mailman id 46864; Mon, 07 Dec 2020 17:22:39 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmKDf-0008KZ-0n for xen-devel@lists.xenproject.org; Mon, 07 Dec 2020 17:22:39 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmKDc-0006KD-L3; Mon, 07 Dec 2020 17:22:36 +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 1kmKDc-0003yK-Aj; Mon, 07 Dec 2020 17:22:36 +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=pZn28BFSU+9QlmOhqKIMuQNIB92uM9LdjTAh4FV+EuI=; b=JsM9ELQtljR6yGWPURl6bfk1IC 7RyI6yInKzbJcGOlIFeYdw+fjF6TRY1y7Ybf2PMonHXsuuwa9ZEhRYaq+mSD4QDmK7vjDbXscuggt ircvSrW80GYI3+gSBCCIJQScVRg+60heFtk5N+rpMsHSDEXe3H+rk2h2e/cRgq7fWG5U=; Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held To: Jan Beulich Cc: Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , Tamas K Lengyel , Petre Ovidiu PIRCALABU , Alexandru Isaila , "xen-devel@lists.xenproject.org" References: <9d7a052a-6222-80ff-cbf1-612d4ca50c2a@suse.com> <17c90493-b438-fbc1-ca10-3bc4d89c4e5e@xen.org> <7a768bcd-80c1-d193-8796-7fb6720fa22a@suse.com> <1a8250f5-ea49-ac3a-e992-be7ec40deba9@xen.org> <269f9a2d-7a8d-cba2-801f-6d3b12f9455f@suse.com> <02a2b77f-27a9-b1b6-1acf-1f136cffdf30@xen.org> <48395363-ea47-9139-011e-233d92581a71@suse.com> <2edfc711-d8d9-4854-94a2-2d9e4d9902ec@xen.org> <381cbc5b-29e8-d84d-0b7c-e84de82bc1a4@suse.com> From: Julien Grall Message-ID: <90ace303-b0a9-7d83-098d-ec01c3b308ad@xen.org> Date: Mon, 7 Dec 2020 17:22:34 +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: <381cbc5b-29e8-d84d-0b7c-e84de82bc1a4@suse.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit On 07/12/2020 08:02, Jan Beulich wrote: > On 04.12.2020 16:09, Julien Grall wrote: >> On 04/12/2020 12:01, Jan Beulich wrote: >>> On 04.12.2020 12:51, Julien Grall wrote: >>>> On 04/12/2020 11:48, Jan Beulich wrote: >>>>> On 04.12.2020 12:28, Julien Grall wrote: >>>>>> On 03/12/2020 10:09, Jan Beulich wrote: >>>>>>> On 02.12.2020 22:10, Julien Grall wrote: >>>>>>>> So shouldn't we handle this issue properly in VM event? >>>>>>> >>>>>>> I suppose that's a question to the VM event folks rather than me? >>>>>> >>>>>> Yes. From my understanding of Tamas's e-mail, they are relying on the >>>>>> monitoring software to do the right thing. >>>>>> >>>>>> I will refrain to comment on this approach. However, given the race is >>>>>> much wider than the event channel, I would recommend to not add more >>>>>> code in the event channel to deal with such problem. >>>>>> >>>>>> Instead, this should be fixed in the VM event code when someone has time >>>>>> to harden the subsystem. >>>>> >>>>> Are effectively saying I should now undo the addition of the >>>>> refcounting, which was added in response to feedback from you? >>>> >>>> Please point out where I made the request to use the refcounting... >>> >>> You didn't ask for this directly, sure, but ... >>> >>>> I pointed out there was an issue with the VM event code. >>> >>> ... this has ultimately led to the decision to use refcounting >>> (iirc there was one alternative that I had proposed, besides >>> the option of doing nothing). >> >> One other option that was discussed (maybe only on security@xen.org) is >> to move the spinlock outside of the structure so it is always allocated. > > Oh, right - forgot about that one, because that's nothing I would > ever have taken on actually carrying out. > >>>> This was latter >>>> analysed as a wider issue. The VM event folks doesn't seem to be very >>>> concerned on the race, so I don't see the reason to try to fix it in the >>>> event channel code. >>> >>> And you won't need the refcount for vpl011 then? >> >> I don't believe we need it for the vpl011 as the spin lock protecting >> the code should always be allocated. The problem today is the lock is >> initialized too late. >> >>> I can certainly >>> drop it again, but it feels odd to go back to an earlier version >>> under the circumstances ... >> >> The code introduced doesn't look necessary outside of the VM event code. >> So I think it would be wrong to merge it if it is just papering over a >> bigger problem. > > So to translate this to a clear course of action: You want me to > go back to the earlier version by dropping the refcounting again? Yes. > (I don't view this as "papering over" btw, but a tiny step towards > a solution.) This is implying that the refcounting is part of the actual solution. I think you can solve it directly in the VM event code without touching the event channel code. Furthermore, I see no point to add code in the common code if the maintainers of the affected subsystem think there code is safe (I don't believe it is). Cheers, -- Julien Grall