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=-11.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY,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 D584BC2D0A3 for ; Thu, 29 Oct 2020 13:08:27 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (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 62CB020732 for ; Thu, 29 Oct 2020 13:08:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="swuMU3k9" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 62CB020732 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Type: Content-Transfer-Encoding:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=hwmSSA2jGkz1XOIFvo9OLaCqTOYXjC4Yu5fvG6SlpxU=; b=swuMU3k9qQIn+v/gX5+bzEJNy BWWmd77dqEV0M4uCXO4blDY1MB1Qy54AjyxdxAhWYveMuUagaRcd/KmqzjKWdAS4vBlDnuQpGN9Qz ZBiVGo0o4OvkTuUGdjiKQAMlYFUkUDg/iXahKTzymylzP9BLxQ9D2nLh0LhzwiBdE7loGZupzFfZE mYtyfI/iYWPJOxPRAbETBrh+ya6dlnJyPqfPDLJzZmBegkj+/lvmpEDxmKxVk2lG4zKx370pSxw0Q mieiHVUcf2Z45gCbTiJ1BU8464CC1cJZchTVku1tHnH8sm5xTYVkyuFoCwz5gsUjr58Pj7UZSl4oE UYx9taV+w==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kY7dv-0000Nt-47; Thu, 29 Oct 2020 13:07:03 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kY7ds-0000NK-Ri for linux-arm-kernel@lists.infradead.org; Thu, 29 Oct 2020 13:07:01 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id ECF40139F; Thu, 29 Oct 2020 06:06:58 -0700 (PDT) Received: from [10.57.54.223] (unknown [10.57.54.223]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E19193F719; Thu, 29 Oct 2020 06:06:57 -0700 (PDT) Subject: Re: [PATCH 4/4] arm64: head: tidy up the Image header definition To: Ard Biesheuvel References: <20201027073209.2897-1-ardb@kernel.org> <20201027073209.2897-5-ardb@kernel.org> <20201028141726.GE28554@willie-the-truck> From: Robin Murphy Message-ID: <74f37b57-e5c0-14ac-5b25-125fc5139112@arm.com> Date: Thu, 29 Oct 2020 13:06:55 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201029_090701_012911_E448C833 X-CRM114-Status: GOOD ( 26.07 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Catalin Marinas , Will Deacon , James Morse , Linux ARM Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 2020-10-29 07:30, Ard Biesheuvel wrote: > On Wed, 28 Oct 2020 at 18:56, Robin Murphy wrote: >> >> On 2020-10-28 14:17, Will Deacon wrote: >>> On Tue, Oct 27, 2020 at 08:32:09AM +0100, Ard Biesheuvel wrote: >>>> Even though support for EFI boot remains entirely optional for arm64, >>>> it is unlikely that we will ever be able to repurpose the image header >>>> fields that the EFI loader relies on, i.e., the magic NOP at offset >>>> 0x0 and the PE header address at offset 0x3c. >>>> >>>> So let's factor out the differences into a 'magic_nop' macro and a local >>>> symbol representing the PE header address, and move the conditional >>>> definitions into efi-header.S, taking into account whether CONFIG_EFI is >>>> enabled or not. >>> >>> How many architectures can claim to have both a "magic nop" and a >>> "mysterious nop", hey? >> >> It's fun 'n'all, but putting my serious hat on for a moment, if the name >> still requires a comment to explain it at point of use, it's not a very >> good name :( >> >> At worst magic_nop is even potentially misleading, since it's not >> necessarily a nop; there's no mention of the implicit dependency on a >> context where the side-effect of executing it wouldn't affect anything >> important. >> >> Could we call the macro itself something clear and self-explanatory like >> efi_signature_insn please? I'm happy for it to be *commented* as "Magic >> NOP" if you want parity with the VDSO :D >> > > Will efi_pseudo_nop do? Again, what's the defining significance of the instruction that this macro stands for - that it does nothing; that it does pseudo-nothing; or that it has a specific signature encoding? I know this probably sounds like bikeshedding to most, but I firmly believe that good, accurate names really do matter :) > Also, do you think it would be better to use an opcode here that has > no architectural side effects, such as a PRFM (literal) instruction? > It is obviously not going to make a difference in practice, but it > always annoyed me that the pseudo NOP is not a NOP. Yeah, it's a shame there's no way to get a guaranteed non-taken conditional branch in A64, and nearly every good candidate for a non-destructive operation with an arbitrary immediate seems to rely on an rt=31 encoding... 'prfm PLIL3STRM, . + 2888' is utterly impenetrable, but should indeed work; 'ccmp x18, #0, #0xd, pl' is probably the least destructive ALU option (only a chance of changing the flags). Robin. >>>> Signed-off-by: Ard Biesheuvel >>>> --- >>>> arch/arm64/kernel/efi-header.S | 43 +++++++++++++++----- >>>> arch/arm64/kernel/head.S | 19 +-------- >>>> 2 files changed, 35 insertions(+), 27 deletions(-) >>>> >>>> diff --git a/arch/arm64/kernel/efi-header.S b/arch/arm64/kernel/efi-header.S >>>> index ddaf57d825b5..7b7ac4316d95 100644 >>>> --- a/arch/arm64/kernel/efi-header.S >>>> +++ b/arch/arm64/kernel/efi-header.S >>>> @@ -7,7 +7,27 @@ >>>> #include >>>> #include >>>> >>>> + .macro magic_nop >>>> +#ifdef CONFIG_EFI >>>> +.L_head: >>>> + /* >>>> + * This add instruction has no meaningful effect except that >>>> + * its opcode forms the magic "MZ" signature required by UEFI. >>>> + */ >>>> + add x13, x18, #0x16 >>> >>> It's probably faster too ;) >>> >>>> +#else >>>> + /* >>>> + * Bootloaders may inspect the opcode at the start of the kernel >>>> + * image to decide if the kernel is capable of booting via UEFI. >>>> + * So put an ordinary NOP here, not the "MZ.." pseudo-nop above. >>>> + */ >>>> + nop >>> >>> Let's just hope nobody was decoding the branch instruction... >>> >>> Acked-by: Will Deacon >will@kernel.org> >>> >>> Will >>> >>> _______________________________________________ >>> linux-arm-kernel mailing list >>> linux-arm-kernel@lists.infradead.org >>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel >>> _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel