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=-12.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,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 226C2C433ED for ; Mon, 3 May 2021 12:27:11 +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 B0721611C9 for ; Mon, 3 May 2021 12:27:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B0721611C9 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.121559.229243 (Exim 4.92) (envelope-from ) id 1ldXf6-000852-5c; Mon, 03 May 2021 12:26:56 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 121559.229243; Mon, 03 May 2021 12:26:56 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1ldXf6-00084v-2L; Mon, 03 May 2021 12:26:56 +0000 Received: by outflank-mailman (input) for mailman id 121559; Mon, 03 May 2021 12:26:54 +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 1ldXf4-00084q-SW for xen-devel@lists.xenproject.org; Mon, 03 May 2021 12:26:54 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 458c1e55-d696-468b-ba1e-5b8563e00a19; Mon, 03 May 2021 12:26:53 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id CFE86AD22; Mon, 3 May 2021 12:26:52 +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: 458c1e55-d696-468b-ba1e-5b8563e00a19 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=1620044812; 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=jTTC7MOxUAEw3uWRU8ucnwrk9K5jqZFX/s22+9DkceY=; b=QhgEdmy+Pxolp72h/ebZutqlj0nBw2YcawA+B3/ds+RPWBErkyHpS01q4uxD+UQdZXsWHu SPmzrtfkhgQ5eGY0sVb0DifcJXhPLDH+v2EmRAzUt/T8O1CEIve5lxUtbx80g/tLEeaURq SwhYb0FYPHV3j30bqs+ye6CZyet2er8= Subject: Re: [PATCH v4 01/12] x86/rtc: drop code related to strict mode To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: Andrew Cooper , Wei Liu , xen-devel@lists.xenproject.org References: <20210420140723.65321-1-roger.pau@citrix.com> <20210420140723.65321-2-roger.pau@citrix.com> From: Jan Beulich Message-ID: <5b06565e-1f2e-3498-c18f-e7eac0042761@suse.com> Date: Mon, 3 May 2021 14:26:51 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 03.05.2021 11:28, Roger Pau Monné wrote: > On Thu, Apr 29, 2021 at 04:53:07PM +0200, Jan Beulich wrote: >> On 20.04.2021 16:07, Roger Pau Monne wrote: >>> --- a/xen/arch/x86/hvm/rtc.c >>> +++ b/xen/arch/x86/hvm/rtc.c >>> @@ -46,15 +46,6 @@ >>> #define epoch_year 1900 >>> #define get_year(x) (x + epoch_year) >>> >>> -enum rtc_mode { >>> - rtc_mode_no_ack, >>> - rtc_mode_strict >>> -}; >>> - >>> -/* This must be in sync with how hvmloader sets the ACPI WAET flags. */ >>> -#define mode_is(d, m) ((void)(d), rtc_mode_##m == rtc_mode_no_ack) >>> -#define rtc_mode_is(s, m) mode_is(vrtc_domain(s), m) >> >> Leaving aside my concerns about this removal, I think some form of >> reference to hvmloader and its respective behavior should remain >> here, presumably in form of a (replacement) comment. > > What about adding a comment in rtc_pf_callback: > > /* > * The current RTC implementation will inject an interrupt regardless > * of whether REG_C has been read since the last interrupt was > * injected. This is why the ACPI WAET 'RTC good' flag must be > * unconditionally set by hvmloader. > */ For one I'm unconvinced this is "must"; I think it is "may". We're producing excess interrupts for an unaware guest, aiui. Presumably most guests can tolerate this, but - second - it may be unnecessary overhead. Which in turn may be why nobody has complained so far, as this sort of overhead my be hard to notice. I also suspect the RTC may not be used very often for generating a periodic interrupt. (I've also not seen the flag named "RTC good" - the ACPI constant is ACPI_WAET_RTC_NO_ACK, for example.) >>> @@ -337,8 +336,7 @@ int pt_update_irq(struct vcpu *v) >>> { >>> if ( pt->pending_intr_nr ) >>> { >>> - /* RTC code takes care of disabling the timer itself. */ >>> - if ( (pt->irq != RTC_IRQ || !pt->priv) && pt_irq_masked(pt) && >>> + if ( pt_irq_masked(pt) && >>> /* Level interrupts should be asserted even if masked. */ >>> !pt->level ) >>> { >> >> I'm struggling to relate this to any other part of the patch. In >> particular I can't find the case where a periodic timer would be >> registered with RTC_IRQ and a NULL private pointer. The only use >> I can find is with a non-NULL pointer, which would mean the "else" >> path is always taken at present for the RTC case (which you now >> change). > > Right, the else case was always taken because as the comment noted RTC > would take care of disabling itself (by calling destroy_periodic_time > from the callback when using strict_mode). When no_ack mode was > implemented this wasn't taken into account AFAICT, and thus the RTC > was never removed from the list even when masked. > > I think with no_ack mode the RTC shouldn't have this specific handling > in pt_update_irq, as it should behave like any other virtual timer. > I could try to split this as a separate bugfix, but then I would have > to teach pt_update_irq to differentiate between strict_mode and no_ack > mode. A fair part of my confusion was about "&& !pt->priv". I've looked back at 9607327abbd3 ("x86/HVM: properly handle RTC periodic timer even when !RTC_PIE"), where this was added. It was, afaict, to cover for hpet_set_timer() passing NULL with RTC_IRQ. Which makes me suspect that be07023be115 ("x86/vhpet: add support for level triggered interrupts") may have subtly broken things. > Would you be fine if the following is added to the commit message > instead: > > "Note that the special handling of the RTC timer done in pt_update_irq > is wrong for the no_ack mode, as the RTC timer callback won't disable > the timer anymore when it detects the guest is not reading REG_C. As > such remove the code as part of the removal of strict_mode, and don't > special case the RTC timer anymore in pt_update_irq." Not sure yet - as per above I'm still not convinced this part of the change is correct. Jan