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 CAF94C433FE for ; Mon, 7 Dec 2020 08:02:37 +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 5A23F22582 for ; Mon, 7 Dec 2020 08:02:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5A23F22582 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.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.45938.81482 (Exim 4.92) (envelope-from ) id 1kmBTU-0006Zt-5n; Mon, 07 Dec 2020 08:02:24 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 45938.81482; Mon, 07 Dec 2020 08:02:24 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmBTU-0006Zm-2U; Mon, 07 Dec 2020 08:02:24 +0000 Received: by outflank-mailman (input) for mailman id 45938; Mon, 07 Dec 2020 08:02:22 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmBTS-0006Zh-Ne for xen-devel@lists.xenproject.org; Mon, 07 Dec 2020 08:02:22 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id d7debd77-597b-4bb4-93cf-dbf0d510bb86; Mon, 07 Dec 2020 08:02:20 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 096ACAC9A; Mon, 7 Dec 2020 08:02:20 +0000 (UTC) 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: d7debd77-597b-4bb4-93cf-dbf0d510bb86 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1607328140; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8jDy2kB4A2gHHrNfM6JAX0OgtL3+GUlngGaCrsMy/xo=; b=YpUOcMadl/5Tzer610sIxp88uh0eVS75GdP8TKuFJzQxFJDskZAOYyAqE3J7TBaJ5dnZtK or7hu1WhKnZQ0MjwxOdal2aEmalLMawu4JScm5KLMBfaPvneTgMvaR+09jeMqwz0XodGJi W2iz37KzUDmSmVHS6Ief3IxLnnhYYgA= Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held To: Julien Grall 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> From: Jan Beulich Message-ID: <381cbc5b-29e8-d84d-0b7c-e84de82bc1a4@suse.com> Date: Mon, 7 Dec 2020 09:02:22 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <2edfc711-d8d9-4854-94a2-2d9e4d9902ec@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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? (I don't view this as "papering over" btw, but a tiny step towards a solution.) Jan