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=-7.3 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,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 43618C433E6 for ; Mon, 22 Feb 2021 07:52:18 +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 C80DD64DDC for ; Mon, 22 Feb 2021 07:52:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C80DD64DDC 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.87786.164988 (Exim 4.92) (envelope-from ) id 1lE60f-0002bD-0n; Mon, 22 Feb 2021 07:52:01 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 87786.164988; Mon, 22 Feb 2021 07:52:00 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lE60e-0002b6-UD; Mon, 22 Feb 2021 07:52:00 +0000 Received: by outflank-mailman (input) for mailman id 87786; Mon, 22 Feb 2021 07:51:59 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lE60d-0002az-GN for xen-devel@lists.xenproject.org; Mon, 22 Feb 2021 07:51:59 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 5efe0135-04ed-480c-bb50-0f53d8d77186; Mon, 22 Feb 2021 07:51:58 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 59536AE36; Mon, 22 Feb 2021 07:51:57 +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: 5efe0135-04ed-480c-bb50-0f53d8d77186 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=1613980317; 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=dlT6ej+2/kuWXutbS9Mejo2sW8ib9b0ySrgZatwo2fA=; b=poNN9bKPZq8LspINsWpFmjpthVz7C0iQApWn5Vi/YYbyL57lP4gQvTT5ewVdtSxIfU0G9l 3+KIStwlkkt+rrXrABCkOrHjQsTaa6M2UkczR3RtyC2DKg31fGv9eX8dNQLnuJUcGvfrBc FZE0/vy0bEf/fHRTXEVlty91IZsp9h8= Subject: Re: [PATCH for-4.15 v2] VMX: use a single, global APIC access page To: Ian Jackson , George Dunlap Cc: "xen-devel@lists.xenproject.org" , Kevin Tian , Julien Grall , Andrew Cooper , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: <24623.61403.440917.434@mariner.uk.xensource.com> From: Jan Beulich Message-ID: Date: Mon, 22 Feb 2021 08:51:59 +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: <24623.61403.440917.434@mariner.uk.xensource.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 19.02.2021 18:05, Ian Jackson wrote: > Jan Beulich writes ("Re: [PATCH v2] VMX: use a single, global APIC access page"): >> While this patch was triggered not just by Julien's observation of >> the early p2m insertion being a problem, but also many earlier >> times of running into this odd code, it is - especially at this >> stage - perhaps a possible option to split the change into just >> the movement of the set_mmio_p2m_entry() invocation and all the >> rest, in order to defer that rest until after 4.15. > > I infer that this contains a bugfix, but perhaps other > changes/improvements too. > > George, I think you're our expert on this refcounting stuff - what do > you think of this ? > > I guess my key question is whether this change will introduce risk by > messing with the complex refcounting machineryt - or remove it by > removing an interaction with the refcounting. If anything, then the latter, but largely neither afaict - there's no change in this regard here at all as far as the guest could affect behavior, due to the page getting inserted as p2m_mmio_direct, and guest_remove_page() having if ( p2mt == p2m_mmio_direct ) { rc = clear_mmio_p2m_entry(d, gmfn, mfn, PAGE_ORDER_4K); goto out_put_gfn; } before any refcounting logic is reached. The removal of interaction is because now the page doesn't get associated with a domain (and hence doesn't become subject to refcounting) at all. The risk of the change stems from going from using a per-domain page to using a single, system-wide one, which indeed was the subject of v1 discussion. In any event the consideration towards splitting the change would cover either concern. Perhaps I should really do so and submit as v3 ... Jan