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=-12.5 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 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 670B4C433DB for ; Mon, 8 Feb 2021 10:48:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1059E64ECD for ; Mon, 8 Feb 2021 10:48:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231404AbhBHKrh (ORCPT ); Mon, 8 Feb 2021 05:47:37 -0500 Received: from mx2.suse.de ([195.135.220.15]:54654 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232520AbhBHKhF (ORCPT ); Mon, 8 Feb 2021 05:37:05 -0500 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=1612780578; 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=5UpmvGJTacd6yUXg2H9+bLosTdiuU5Va+P2BDP3oi4Y=; b=QrwXNShCzDyYAZA4BYXeiZBCyfJGNbF/wzlZPiEU3K8Ie6xvQEI1yjQpnzynJRqYeVDDfU KODbn7VjygcHyxQAPUiPYawbNC9JOaoFJqTVptTceuTM4hTUTnHQJIaAqTaLykBtTgvKb5 g5ANxsDslCGRmAKiMXbh/ZysXVTk3KU= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 5FFD2AC6E; Mon, 8 Feb 2021 10:36:18 +0000 (UTC) Subject: Re: [PATCH 6/7] xen/evtch: use smp barriers for user event ring To: Andrew Cooper Cc: Boris Ostrovsky , Stefano Stabellini , Juergen Gross , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org References: <20210206104932.29064-1-jgross@suse.com> <20210206104932.29064-7-jgross@suse.com> <2d354cad-3413-a416-0bc1-01d03e1f41cd@citrix.com> From: Jan Beulich Message-ID: Date: Mon, 8 Feb 2021 11:36:17 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08.02.2021 11:23, Andrew Cooper wrote: > On 08/02/2021 09:50, Jan Beulich wrote: >> On 08.02.2021 10:44, Andrew Cooper wrote: >>> On 06/02/2021 10:49, Juergen Gross wrote: >>>> The ring buffer for user events is used in the local system only, so >>>> smp barriers are fine for ensuring consistency. >>>> >>>> Reported-by: Andrew Cooper >>>> Signed-off-by: Juergen Gross >>> These need to be virt_* to not break in UP builds (on non-x86). >> Initially I though so, too, but isn't the sole vCPU of such a >> VM getting re-scheduled to a different pCPU in the hypervisor >> an implied barrier anyway? > > Yes, but that isn't relevant to why UP builds break. > > smp_*() degrade to compiler barriers in UP builds, and while that's > mostly fine for x86 read/write, its not fine for ARM barriers. Hmm, I may not know enough of Arm's memory model - are you saying Arm CPUs aren't even self-coherent, i.e. later operations (e.g. the consuming of ring contents) won't observe earlier ones (the updating of ring contents) when only a single physical CPU is involved in all of this? (I did mention the hypervisor level context switch simply because that's the only way multiple CPUs can get involved.) Jan