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_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 B2515C433E0 for ; Tue, 23 Feb 2021 16:13:37 +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 5E19C64E20 for ; Tue, 23 Feb 2021 16:13:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5E19C64E20 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.88885.167280 (Exim 4.92) (envelope-from ) id 1lEaJQ-0006Yd-HQ; Tue, 23 Feb 2021 16:13:24 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 88885.167280; Tue, 23 Feb 2021 16:13:24 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lEaJQ-0006YW-Ec; Tue, 23 Feb 2021 16:13:24 +0000 Received: by outflank-mailman (input) for mailman id 88885; Tue, 23 Feb 2021 16:13:23 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lEaJP-0006YR-3M for xen-devel@lists.xenproject.org; Tue, 23 Feb 2021 16:13:23 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 4632d9a5-f33d-417d-bfff-3da4782474cc; Tue, 23 Feb 2021 16:13:22 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 5F5ABAB95; Tue, 23 Feb 2021 16:13:21 +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: 4632d9a5-f33d-417d-bfff-3da4782474cc 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=1614096801; 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=Q1DLj/+4Nqge5qsuEp8WvxJqf7aK5Y4qbufSexgLYQE=; b=AXdNyMaVqxaiRkJ5YzPlYElPK9J5ze8OfJNMRgvqLNput3TLiD2zzbl5RgWq6oFGtkzHyl kd/6Lfo7UqXbeaVLHaYEo8cQyf6h2srlaPxC4DaGLr92F8DO2iSMBSSm1BhCQ8QuevoQg5 8s3bkkGhpIHXXOJUYq1xCXf+5KxKtgY= Subject: Re: [PATCH v2 8/8] x86/PV: use get_unsafe() instead of copy_from_unsafe() To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Wei Liu , George Dunlap , Ian Jackson References: <0a59ae2f-448e-610d-e8a2-a7c3f9f3918f@suse.com> <76207250-1372-e7ab-2d03-b46020a7906b@suse.com> From: Jan Beulich Message-ID: <4fdb5952-6196-3a79-1306-e65d75e495d2@suse.com> Date: Tue, 23 Feb 2021 17:13:21 +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: 8bit On 23.02.2021 16:37, Roger Pau Monné wrote: > On Tue, Feb 23, 2021 at 04:25:00PM +0100, Jan Beulich wrote: >> On 23.02.2021 12:59, Roger Pau Monné wrote: >>> On Wed, Feb 17, 2021 at 09:23:33AM +0100, Jan Beulich wrote: >>>> The former expands to a single (memory accessing) insn, which the latter >>>> does not guarantee. Yet we'd prefer to read consistent PTEs rather than >>>> risking a split read racing with an update done elsewhere. >>>> >>>> Signed-off-by: Jan Beulich >>> >>> Reviewed-by: Roger Pau Monné >>> >>> Albeit I wonder why the __builtin_constant_p check done in >>> copy_from_unsafe is not enough to take the get_unsafe_size branch in >>> there. Doesn't sizeof(l{1,2}_pgentry_t) qualify as a built time >>> constant? >>> >>> Or the fact that n it's a parameter to an inline function hides this, >>> in which case the __builtin_constant_p is pointless? >> >> Without (enough) optimization, __builtin_constant_p() may indeed >> yield false in such cases. But that wasn't actually what I had >> in mind when making this change (and the original similar on in >> shadow code). Instead, at the time I made the shadow side change, >> I had removed this optimization from the new function flavors. >> With that removal, things are supposed to still be correct - it's >> an optimization only, after all. Meanwhile the optimizations are >> back, so there's no immediate problem as long as the optimizer >> doesn't decide to out-of-line the function invocations (we >> shouldn't forget that even always_inline is not a guarantee for >> inlining to actually occur). > > I'm fine with you switching those use cases to get_unsafe, but I think > the commit message should be slightly adjusted to notice that > copy_from_unsafe will likely do the right thing, but that it's simply > clearer to call get_unsafe directly, also in case copy_from_unsafe > gets changed in the future to drop the get_unsafe paths. How about this then? "The former expands to a single (memory accessing) insn, which the latter does not guarantee (the __builtin_constant_p() based switch() statement there is just an optimization). Yet we'd prefer to read consistent PTEs rather than risking a split read racing with an update done elsewhere." Jan