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 F234FC433B4 for ; Mon, 19 Apr 2021 16:01:07 +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 A635C60E0C for ; Mon, 19 Apr 2021 16:01:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A635C60E0C 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.113094.215513 (Exim 4.92) (envelope-from ) id 1lYWKY-0002Kk-7r; Mon, 19 Apr 2021 16:00:58 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 113094.215513; Mon, 19 Apr 2021 16:00:58 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lYWKY-0002Kd-4q; Mon, 19 Apr 2021 16:00:58 +0000 Received: by outflank-mailman (input) for mailman id 113094; Mon, 19 Apr 2021 16:00:56 +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 1lYWKW-0002KS-GA for xen-devel@lists.xenproject.org; Mon, 19 Apr 2021 16:00:56 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 6ec992a8-9bcc-4739-bfc7-1aab5a54da46; Mon, 19 Apr 2021 16:00:55 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 20672AF35; Mon, 19 Apr 2021 16:00:55 +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: 6ec992a8-9bcc-4739-bfc7-1aab5a54da46 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=1618848055; 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=oIuELSFZXR96S1oKPJlsusqOvaata/PLNROEUBemQI0=; b=vWT7MGFcfjVdSqxs5eXKGvUIxTHlZMJ6+Uv/Uhsc/CvV45jVWRyyMVHyC5PW+B/0lCu8K8 PTz9NbmN+SgpY3Z9KLKv5SQ0+nbdto3eByLg6ayAh1R6pRf0hPjGMRTwp8lfzImDnubFNu YY8RPxg6lqngkqoXKLsgSxHtRwwxGRk= Subject: Re: [PATCH] x86/shim: Simplify compat handling in write_start_info() To: Andrew Cooper Cc: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Wei Liu , Xen-devel References: <20210419144524.32608-1-andrew.cooper3@citrix.com> From: Jan Beulich Message-ID: <398b912d-d19f-8321-815a-362c6da6d70d@suse.com> Date: Mon, 19 Apr 2021 18:00:54 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 19.04.2021 17:57, Andrew Cooper wrote: > On 19/04/2021 16:55, Jan Beulich wrote: >> On 19.04.2021 16:45, Andrew Cooper wrote: >>> Factor out a compat boolean to remove the lfence overhead from multiple >>> is_pv_32bit_domain() calls. >>> >>> For a compat guest, the upper 32 bits of rdx are zero, so there is no need to >>> have any conditional logic at all when mapping the start info page. >> Iirc the contents of the upper halves hold unspecified contents after >> a switch from compat to 64-bit mode. Therefore only with this part of >> the change dropped ... > > But we're shim, so will never ever mix compat and non-compat guests. That's not the point: A compat guest will still cause the CPU to transition back and forth between 64-bit and compat modes. It is this transitioning which leaves the upper halves of all GPRs in undefined state (even if in reality a CPU would likely need to go through extra hoops to prevent them from being zero if they were written to in compat mode). Jan