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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 8DCCFC433EF for ; Fri, 10 Dec 2021 14:45:24 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.243980.422114 (Exim 4.92) (envelope-from ) id 1mvh8p-0003OY-Ij; Fri, 10 Dec 2021 14:44:55 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 243980.422114; Fri, 10 Dec 2021 14:44:55 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1mvh8p-0003OR-FG; Fri, 10 Dec 2021 14:44:55 +0000 Received: by outflank-mailman (input) for mailman id 243980; Fri, 10 Dec 2021 14:44:54 +0000 Received: from se1-gles-sth1-in.inumbo.com ([159.253.27.254] helo=se1-gles-sth1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1mvh8o-0003OH-3N for xen-devel@lists.xenproject.org; Fri, 10 Dec 2021 14:44:54 +0000 Received: from ppsw-42.csi.cam.ac.uk (ppsw-42.csi.cam.ac.uk [131.111.8.142]) by se1-gles-sth1.inumbo.com (Halon) with ESMTPS id bbe59301-59c7-11ec-9d12-4777fae47e2b; Fri, 10 Dec 2021 15:44:53 +0100 (CET) Received: from hades.srcf.societies.cam.ac.uk ([131.111.179.67]:36080) by ppsw-42.csi.cam.ac.uk (ppsw.cam.ac.uk [131.111.8.138]:25) with esmtps (TLS1.2:ECDHE-RSA-AES256-GCM-SHA384:256) id 1mvh8k-000JOB-7Q (Exim 4.95) (return-path ); Fri, 10 Dec 2021 14:44:50 +0000 Received: from [192.168.1.10] (host-92-12-61-86.as13285.net [92.12.61.86]) (Authenticated sender: amc96) by hades.srcf.societies.cam.ac.uk (Postfix) with ESMTPSA id 072851FBFC; Fri, 10 Dec 2021 14:44:50 +0000 (GMT) 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: bbe59301-59c7-11ec-9d12-4777fae47e2b X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: https://help.uis.cam.ac.uk/email-scanner-virus Message-ID: Date: Fri, 10 Dec 2021 14:44:49 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.2 Content-Language: en-GB To: Jan Beulich , Andrew Cooper Cc: Stefano Stabellini , Wei Liu , Julien Grall , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Xen-devel References: <20211126123446.32324-1-andrew.cooper3@citrix.com> <20211126123446.32324-9-andrew.cooper3@citrix.com> From: Andrew Cooper Subject: Re: [PATCH 08/65] xen: Annotate fnptr targets from acpi_table_parse() In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 06/12/2021 08:36, Jan Beulich wrote: > On 26.11.2021 13:33, Andrew Cooper wrote: >> --- a/xen/arch/x86/hvm/dom0_build.c >> +++ b/xen/arch/x86/hvm/dom0_build.c > Elsewhere in this file we have > > rc = map ? map_mmio_regions(d, _gfn(pfn), nr_pages, _mfn(pfn)) > : unmap_mmio_regions(d, _gfn(pfn), nr_pages, _mfn(pfn)); > > which has been in this shape only as of commit e3b418ac4919 > ("x86/pvh-dom0: Remove unnecessary function pointer call from > modify_identity_mmio()"). Aren't we relying on the compiler not > transforming this back into the earlier > > rc = (map ? map_mmio_regions : unmap_mmio_regions) > (d, _gfn(pfn), nr_pages, _mfn(pfn)); > > ? That old code was especially dumb even before retpoline.  See also the damage caused by c/s 245a320ce2. Yes, we are relying on the compiler not to do transformations behind our backs, but it won't of its own accord. > And aren't we further relying on the compiler not transforming direct > calls into indirect ones for other reasons (I recall Microsoft's compiler > being pretty aggressive about this when the same function was called > more than once in close succession, it at least certain past versions)? That sounds like a broken compiler. There are legal cases where a direct call has to turn into an indirect one, and that's when we need to traverse more than disp32 distance. But without going to a larger mcmodel, we'd get linker errors before that becomes a problem, because R_X86_64_PLT32 relocations can't be retrofitted into an indirect call at link time. > Is the widened effect of the annotation intended to also guarantee that > indirect calls will not be produced by the compiler for any reason when > the annotation is absent on a targeted function's declaration? That would be one for the clang and gcc developers. I don't see a plausible problem here. ~Andrew