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=-5.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 942F3C43331 for ; Mon, 30 Mar 2020 14:28:30 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id E4E5F20675 for ; Mon, 30 Mar 2020 14:28:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="XNblZkoN" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E4E5F20675 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18301-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 27716 invoked by uid 550); 30 Mar 2020 14:28:23 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 27681 invoked from network); 30 Mar 2020 14:28:22 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585578490; bh=85ALpUpc/X35vGswsxKBqkjPj2xZrOJvnB1DddBDMZA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=XNblZkoNvj7W1OGCUQTb3NYn/of4v49KGSVP5Fx5FXofPYzIQq/HG8rGv8cGqxOMV n64oS5KtVdYanc5t7/fUA+qjFQwDE1PD4mD9fPEWNohfNr2jkZuqTFi17PUO8dCjSJ l2czSpN9T4OZTp0iJoQgXk+kgUGSIePZe6ceDB3o= Date: Mon, 30 Mar 2020 15:28:06 +0100 From: Will Deacon To: Ard Biesheuvel Cc: Linux ARM , kernel-hardening@lists.openwall.com, Catalin Marinas , Mark Rutland Subject: Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature Message-ID: <20200330142805.GA11312@willie-the-truck> References: <20200329141258.31172-1-ardb@kernel.org> <20200330135121.GD10633@willie-the-truck> <20200330140441.GE10633@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) On Mon, Mar 30, 2020 at 04:22:24PM +0200, Ard Biesheuvel wrote: > On Mon, 30 Mar 2020 at 16:04, Will Deacon wrote: > > > > On Mon, Mar 30, 2020 at 03:53:04PM +0200, Ard Biesheuvel wrote: > > > On Mon, 30 Mar 2020 at 15:51, Will Deacon wrote: > > > > > > > > On Sun, Mar 29, 2020 at 04:12:58PM +0200, Ard Biesheuvel wrote: > > > > > When CONFIG_DEBUG_ALIGN_RODATA is enabled, kernel segments mapped with > > > > > different permissions (r-x for .text, r-- for .rodata, rw- for .data, > > > > > etc) are rounded up to 2 MiB so they can be mapped more efficiently. > > > > > In particular, it permits the segments to be mapped using level 2 > > > > > block entries when using 4k pages, which is expected to result in less > > > > > TLB pressure. > > > > > > > > > > However, the mappings for the bulk of the kernel will use level 2 > > > > > entries anyway, and the misaligned fringes are organized such that they > > > > > can take advantage of the contiguous bit, and use far fewer level 3 > > > > > entries than would be needed otherwise. > > > > > > > > > > This makes the value of this feature dubious at best, and since it is not > > > > > enabled in defconfig or in the distro configs, it does not appear to be > > > > > in wide use either. So let's just remove it. > > > > > > > > > > Signed-off-by: Ard Biesheuvel > > > > > --- > > > > > arch/arm64/Kconfig.debug | 13 ------------- > > > > > arch/arm64/include/asm/memory.h | 12 +----------- > > > > > drivers/firmware/efi/libstub/arm64-stub.c | 8 +++----- > > > > > 3 files changed, 4 insertions(+), 29 deletions(-) > > > > > > > > Acked-by: Will Deacon > > > > > > > > But I would really like to go a step further and rip out the block mapping > > > > support altogether so that we can fix non-coherent DMA aliases: > > > > > > > > https://lore.kernel.org/lkml/20200224194446.690816-1-hch@lst.de > > > > > > > > > > I'm not sure I follow - is this about mapping parts of the static > > > kernel Image for non-coherent DMA? > > > > Sorry, it's not directly related to your patch, just that if we're removing > > options relating to kernel mappings then I'd be quite keen on effectively > > forcing page-granularity on the linear map, as is currently done by default > > thanks to RODATA_FULL_DEFAULT_ENABLED, so that we can nobble cacheable > > aliases for non-coherent streaming DMA mappings by hooking into Christoph's > > series above. > > > > Right. I don't remember seeing any complaints about > RODATA_FULL_DEFAULT_ENABLED, but maybe it's too early for that. > > > This series just reminded me of it because it's another > > "off-by-default-behaviour-for-block-mappings-probably-because-of-performance- > > but-never-actually-measured" type of thing which really just gets in the > > way. > > > > Well, even though I agree that the lack of actual numbers is a bit > disturbing here, I'd hate to penalize all systems even more than they > already are (due to ARCH_KMALLOC_MINALIGN == ARCH_DMA_MINALIGN) by > adding another workaround that is only needed on devices that have > non-coherent masters. Fair enough, but I'd still like to see some numbers. If they're compelling, then we could explore something like CONFIG_OF_DMA_DEFAULT_COHERENT, but that doesn't really help the kconfig maze :( Will