From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 618AC39A063; Tue, 7 Apr 2026 08:33:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775550846; cv=none; b=q9LTNegy/snueT4Sl1AYWv59TAsVK8aWxlsG0wkPMByUgVyIfoy+78aRJRDDeQf43dg+w4UEn0cgce6o7cb6R8fMTqv6cS8m1g3aHU7Wn8HtvgBmT6m6vgoyDOgE2/0IF/vq1OJDDDTTr699YRStgUIOdM2kixDgNTl0SEaWi4o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775550846; c=relaxed/simple; bh=Heg4YQbV1vvgUn0TgfKMiao62MWpbj3co/ZqYPpLlFM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Bmh0qUA50kuqdXTGE5u8LDysSaPVloYy8L83I5O2J9t2cpQ9KkxtDNqRvD0rwRqoVazTmN6aQD1v98Jqxfrx6hvnwGrN+U1ym47zyFeiBOkRQWtlPWWiJonG+BCzqObkgxupkmUv/Y2DovKAOc2x37SnAjWapAl/GuopS7drF4s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=BeOvKBOw; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="BeOvKBOw" 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 1C45E1BB0; Tue, 7 Apr 2026 01:33:48 -0700 (PDT) Received: from [10.57.87.42] (unknown [10.57.87.42]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 27AA33F641; Tue, 7 Apr 2026 01:33:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1775550833; bh=Heg4YQbV1vvgUn0TgfKMiao62MWpbj3co/ZqYPpLlFM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=BeOvKBOwcfSrrgOOwLTx2bedDd9LnW/QQ2dazXB+DPDeacQE/pprpv5KjsFESUOp7 QAEEweOhufhnbL9FV8Fq+lLlX6dX9T/DlPrmxQ6hnW14JoYODNIAQmtXeIsWW89I5K R1kdOEEy+hZf4eWOH8LvTYDjyXg4riRLxA3N6RwA= Message-ID: <160ec79a-f842-421a-bfde-5b4da32b3b4e@arm.com> Date: Tue, 7 Apr 2026 09:33:50 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/3] arm64: mm: Fix rodata=full block mapping support for realm guests Content-Language: en-GB To: Catalin Marinas Cc: Will Deacon , "David Hildenbrand (Arm)" , Dev Jain , Yang Shi , Suzuki K Poulose , Jinjiang Tu , Kevin Brodsky , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260330161705.3349825-1-ryan.roberts@arm.com> <20260330161705.3349825-2-ryan.roberts@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 02/04/2026 21:43, Catalin Marinas wrote: > On Mon, Mar 30, 2026 at 05:17:02PM +0100, Ryan Roberts wrote: >> int split_kernel_leaf_mapping(unsigned long start, unsigned long end) >> { >> int ret; >> >> - /* >> - * !BBML2_NOABORT systems should not be trying to change permissions on >> - * anything that is not pte-mapped in the first place. Just return early >> - * and let the permission change code raise a warning if not already >> - * pte-mapped. >> - */ >> - if (!system_supports_bbml2_noabort()) >> - return 0; >> - >> /* >> * If the region is within a pte-mapped area, there is no need to try to >> * split. Additionally, CONFIG_DEBUG_PAGEALLOC and CONFIG_KFENCE may >> * change permissions from atomic context so for those cases (which are >> * always pte-mapped), we must not go any further because taking the >> - * mutex below may sleep. >> + * mutex below may sleep. Do not call force_pte_mapping() here because >> + * it could return a confusing result if called from a secondary cpu >> + * prior to finalizing caps. Instead, linear_map_requires_bbml2 gives us >> + * what we need. >> */ >> - if (force_pte_mapping() || is_kfence_address((void *)start)) >> + if (!linear_map_requires_bbml2 || is_kfence_address((void *)start)) >> return 0; >> >> + if (!system_supports_bbml2_noabort()) { >> + /* >> + * !BBML2_NOABORT systems should not be trying to change >> + * permissions on anything that is not pte-mapped in the first >> + * place. Just return early and let the permission change code >> + * raise a warning if not already pte-mapped. >> + */ >> + if (system_capabilities_finalized()) >> + return 0; >> + >> + /* >> + * Boot-time: split_kernel_leaf_mapping_locked() allocates from >> + * page allocator. Can't split until it's available. >> + */ >> + if (WARN_ON(!page_alloc_available)) >> + return -EBUSY; >> + >> + /* >> + * Boot-time: Started secondary cpus but don't know if they >> + * support BBML2_NOABORT yet. Can't allow splitting in this >> + * window in case they don't. >> + */ >> + if (WARN_ON(num_online_cpus() > 1)) >> + return -EBUSY; >> + } > > I think sashiko is over cautions here > (https://sashiko.dev/#/patchset/20260330161705.3349825-1-ryan.roberts@arm.com) > but it has a somewhat valid point from the perspective of > num_online_cpus() semantics. We have have num_online_cpus() == 1 while > having a secondary CPU just booted and with its MMU enabled. I don't > think we can have any asynchronous tasks running at that point to > trigger a spit though. Even async_init() is called after smp_init(). Yes I saw the Sashiko report, but we had previously had a (private) discussion where I thought we had already concluded that this approach is safe in practice due to the way that the boot cpu brings the secondaries online. > > An option may be to attempt cpus_read_trylock() as this lock is taken by > _cpu_up(). If it fails, return -EBUSY, otherwise check num_online_cpus() > and unlock (and return -EBUSY if secondaries already started). That sounds neat; I could dig deeper and have a go at something like this if you want? > > Another thing I couldn't get my head around - IIUC is_realm_world() > won't return true for map_mem() yet (if in a realm). Can we have realms > on hardware that does not support BBML2_NOABORT? We may not have > configuration with rodata_full set (it should be complementary to realm > support). My understanding is that this is a pre-existing (and known) bug. It's not related to the "map linear map by large leaves and split dynamically" feature so wasn't attempting to fix it. I had heard that in practice all FEAT_RME systems should support FEAT_BBML3 which would solve the problem. Not sure how true that is though. > > I'll add the patches to for-next/core to give them a bit of time in > -next but let's see next week if we ignore this (with an updated > comment) or we try to avoid the issue altogether. > Thanks, Ryan