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=-3.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 2EA07C433DF for ; Wed, 15 Jul 2020 14:52:18 +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 F1222206E9 for ; Wed, 15 Jul 2020 14:52:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=citrix.com header.i=@citrix.com header.b="EIVyseaY" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F1222206E9 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=citrix.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jvilL-0001dj-AY; Wed, 15 Jul 2020 14:51:59 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jvilJ-0001de-NU for xen-devel@lists.xenproject.org; Wed, 15 Jul 2020 14:51:57 +0000 X-Inumbo-ID: b9cfe394-c6aa-11ea-b7bb-bc764e2007e4 Received: from esa1.hc3370-68.iphmx.com (unknown [216.71.145.142]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id b9cfe394-c6aa-11ea-b7bb-bc764e2007e4; Wed, 15 Jul 2020 14:51:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=citrix.com; s=securemail; t=1594824716; h=date:from:to:cc:subject:message-id:references: mime-version:content-transfer-encoding:in-reply-to; bh=V9kxh95A7KNWJvFvSiPuEvHRhmMVhybG14SBrb4QY+E=; b=EIVyseaYNLTMjGKi7B4bXp5wvzfV9PtV2NZlWaPXeqoe8RuGwz3jq7Uh HVa/Bp6RMETHU1Lz+51juj7+UuE0CEQqKLHy2GgapUe+T5SBD8oc6CAKv 5EtbOuLWr/pYGSYwhIqeUbjVHp+osvW1Vbg2LkvGLP5+dxwi4aGsjnPL9 s=; Authentication-Results: esa1.hc3370-68.iphmx.com; dkim=none (message not signed) header.i=none IronPort-SDR: f1sF7yFgansxJzTdNSfXVLpbKRJOxUUfXhqwzcRT1UUeoMqh5ONM+bti1/z7HeNB2mKLtOfNgc /1L4DlYsmLCrNwEcPZVKUeHbv3lL4eXZYTc6U85X5nsjvNmPMVflVG5snfjeTclJ3JcUz0O0Da 6bKy7af6HyRYQEPbBbYZc8OlICyCRx6GixQJg5CYtyLP6zoUAYr/mBNu+BS4MBetmfmrwGmQL7 o7JTOYPmqaeSAt7SwObxy8fd5iiIBeGy5a5UJZMoD7wwbN4jMVb4sM51Q03ZzrtlEAWQZRLxFT 9Xs= X-SBRS: 2.7 X-MesageID: 22761476 X-Ironport-Server: esa1.hc3370-68.iphmx.com X-Remote-IP: 162.221.158.21 X-Policy: $RELAYED X-IronPort-AV: E=Sophos;i="5.75,355,1589256000"; d="scan'208";a="22761476" Date: Wed, 15 Jul 2020 16:51:44 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Jan Beulich Subject: Re: [PATCH v3 1/2] x86: restore pv_rtc_handler() invocation Message-ID: <20200715145144.GA7191@Air-de-Roger> References: <5426dd6f-50cd-dc23-5c6b-0ab631d98d38@suse.com> <7dd4b668-06ca-807a-9cc1-77430b2376a8@suse.com> <20200715121347.GY7191@Air-de-Roger> <2b9de0fd-5973-ed66-868c-ffadca83edf3@suse.com> <20200715133217.GZ7191@Air-de-Roger> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: AMSPEX02CAS02.citrite.net (10.69.22.113) To AMSPEX02CL02.citrite.net (10.69.22.126) X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: "xen-devel@lists.xenproject.org" , Paul Durrant , Wei Liu , Andrew Cooper Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On Wed, Jul 15, 2020 at 03:51:17PM +0200, Jan Beulich wrote: > On 15.07.2020 15:32, Roger Pau Monné wrote: > > On Wed, Jul 15, 2020 at 02:36:49PM +0200, Jan Beulich wrote: > >> On 15.07.2020 14:13, Roger Pau Monné wrote: > >>> On Wed, Jul 15, 2020 at 01:56:47PM +0200, Jan Beulich wrote: > >>>> @@ -1160,6 +1162,14 @@ void rtc_guest_write(unsigned int port, > >>>> case RTC_PORT(1): > >>>> if ( !ioports_access_permitted(currd, RTC_PORT(0), RTC_PORT(1)) ) > >>>> break; > >>>> + > >>>> + spin_lock_irqsave(&rtc_lock, flags); > >>>> + hook = pv_rtc_handler; > >>>> + spin_unlock_irqrestore(&rtc_lock, flags); > >>> > >>> Given that clearing the pv_rtc_handler variable in handle_rtc_once is > >>> not done while holding the rtc_lock, I'm not sure there's much point > >>> in holding the lock here, ie: just doing something like: > >>> > >>> hook = pv_rtc_handler; > >>> if ( hook ) > >>> hook(currd->arch.cmos_idx & 0x7f, data); > >>> > >>> Should be as safe as what you do. > >> > >> No, the compiler is free to eliminate the local variable and read > >> the global one twice (and it may change contents in between) then. > >> I could use ACCESS_ONCE() or read_atomic() here, but then it would > >> become quite clear that at the same time ... > >> > >>> We also assume that setting pv_rtc_handler to NULL is an atomic > >>> operation. > >> > >> ... this (which isn't different from what we do elsewhere, and we > >> really can't fix everything at the same time) ought to also become > >> ACCESS_ONCE() (or write_atomic()). > >> > >> A compromise might be to use barrier() in place of the locking for > >> now ... > > > > Oh, right. Didn't realize you did it in order to prevent > > optimizations. Using the lock seems also quite weird IMO, so I'm not > > sure it's much better than just using ACCESS_ONCE (or a barrier). > > Anyway, I don't want to delay this any longer, so: > > > > Reviewed-by: Roger Pau Monné > > Thanks. > > > Feel free to change to ACCESS_ONCE or barrier if you think it's > > clearer. > > I did so (also on the writer side), not the least based on guessing > what Andrew would presumably have preferred. Thanks! Sorry I might be pedantic, but is the ACCESS_ONCE on the write side actually required? I'm not sure I see what ACCESS_ONCE protects against in handle_rtc_once. Roger.