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=-12.2 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,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 7095FC433ED for ; Mon, 3 May 2021 14:58:22 +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 137D8611EE for ; Mon, 3 May 2021 14:58:22 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 137D8611EE 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.121675.229484 (Exim 4.92) (envelope-from ) id 1lda1S-0005ul-V2; Mon, 03 May 2021 14:58:10 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 121675.229484; Mon, 03 May 2021 14:58:10 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lda1S-0005ue-Rv; Mon, 03 May 2021 14:58:10 +0000 Received: by outflank-mailman (input) for mailman id 121675; Mon, 03 May 2021 14:58:09 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lda1R-0005uZ-KE for xen-devel@lists.xenproject.org; Mon, 03 May 2021 14:58:09 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id b1f3d413-0433-4dac-8db6-72e2f8fa8ff6; Mon, 03 May 2021 14:58:08 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A5366AF0E; Mon, 3 May 2021 14:58:07 +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: b1f3d413-0433-4dac-8db6-72e2f8fa8ff6 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=1620053887; 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=qgAJdx89/lcuYzZsPoRl0Y9RMMFmqP8xIi4lpsqcidI=; b=VlvGFXCw36btspImOnKgGIJJnvHd1omnXJtSHxOVSrCvI0+MxQ9ZqFnf9bwXBjPk8yQ7Do 7M7iDYs3cUWX+woLZkPwz16p2hT3tohWDV+qd9v8MWWMw5wiFppKrprbFyaQbwQtFntf1B WSvzJ79rYyLtpz25DVDi0PaLu8hlGKU= 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> <5b06565e-1f2e-3498-c18f-e7eac0042761@suse.com> From: Jan Beulich Message-ID: Date: Mon, 3 May 2021 16:58:07 +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 16:47, Roger Pau Monné wrote: > On Mon, May 03, 2021 at 02:26:51PM +0200, Jan Beulich wrote: >> 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: >> (I've also not seen the >> flag named "RTC good" - the ACPI constant is ACPI_WAET_RTC_NO_ACK, for >> example.) > > I'm reading the WAET spec as published my Microsoft: > > http://msdn.microsoft.com/en-us/windows/hardware/gg487524.aspx > > Where the flag is listed as 'RTC good'. Maybe that's outdated now? > Seems to be the official source for the specification from > https://uefi.org/acpi. Well, I guess the wording wasn't used for the constant's name because the RTC isn't "bad" otherwise? >>>>> @@ -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 think you meant "|| !pt->priv"? Oops, indeed. >> 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. > > That's tricky, as hpet_set_timer hardcodes 8 instead of using RTC_IRQ > which makes it really easy to miss. > >> Which makes me suspect that >> be07023be115 ("x86/vhpet: add support for level triggered interrupts") >> may have subtly broken things. > > Right - as that would have made the RTC irq when generated from the > HPET no longer be suspended when masked (as pt->priv would no longer > be NULL). Could be fixed with: > > diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c > index ca94e8b4538..f2cbd12f400 100644 > --- a/xen/arch/x86/hvm/hpet.c > +++ b/xen/arch/x86/hvm/hpet.c > @@ -318,7 +318,8 @@ static void hpet_set_timer(HPETState *h, unsigned int tn, > hpet_tick_to_ns(h, diff), > oneshot ? 0 : hpet_tick_to_ns(h, h->hpet.period[tn]), > irq, timer_level(h, tn) ? hpet_timer_fired : NULL, > - (void *)(unsigned long)tn, timer_level(h, tn)); > + timer_level(h, tn) ? (void *)(unsigned long)tn : NULL, > + timer_level(h, tn)); > } > > static inline uint64_t hpet_fixup_reg( > > Passing again NULL as the callback private data for edge triggered > interrupts. Right, plus perhaps at the same time replacing the hardcoded 8. >>> 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. > > I believe part of this handling is kind of bogus - for example I'm > unsure Xen should account masked interrupt injections as missed ticks. > A guest might decide to mask it's interrupt source for whatever > reason, and then it shouldn't receive a flurry of interrupts when > unmasked. Ie: missed ticks should only be accounted for interrupts > that should have been delivered but the guest wasn't scheduled. I > think such model would also simplify some of the logic that we > currently have. > > In fact I have a patch on top of this current series which I haven't > posted yet that does implement this new mode of not accounting masked > interrupts as missed ticks to the delivered later. This may be problematic: Iirc one of the goals of this mode is to cover for the case where a guest simply doesn't get around to unmasking the IRQ until the next one occurs. Yes, it feels bogus, but I'm not sure it can be done away with. I also can't seem to be able to think of a heuristic by which the two scenarios could be told apart halfway reliably. Jan