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 04CE625C832 for ; Mon, 12 May 2025 13:07:46 +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=1747055269; cv=none; b=pK4F/1NtfxBcsR5oAZefbCIaFce0Kqxqxn4laLOyTrR2mRa/WJN1DmEi39YAg6LE9WjxYXJBaI+luxffB3CLGyZyP1hwhXP0lVazyajmegQ3IDVPGbYAiH9iQ/BMLd0i1sSsISrfLk7ViivaL5eR9jJZacPP6rDEvaJJwIYaCS4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747055269; c=relaxed/simple; bh=1dC9tqxjkLDhSyRziAIuOV7EgJfqIP8QHeVCN6LKOMs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kbFVfAKWpCHVzvuGAsM34Z+c6a6pNRfKc4AlymYsasWZVIb8roU4q0cIyxsff4qdoQX/bSlLObDK2+p8wiF9VzC1GD7ZICRzAS5RjVM3oefQ609FlZPh8SldKzesMLYRoSYhwB6Rkb/81Zmfd/iZbm3BuAhDFf7UScSbgF35vTQ= 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; 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 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 2706C150C; Mon, 12 May 2025 06:07:35 -0700 (PDT) Received: from [10.57.90.222] (unknown [10.57.90.222]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7962D3F5A1; Mon, 12 May 2025 06:07:41 -0700 (PDT) Message-ID: <015746d7-ca46-4978-a441-09fba781fdd4@arm.com> Date: Mon, 12 May 2025 14:07:39 +0100 Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RESEND PATCH v6 1/3] arm64: Add BBM Level 2 cpu feature Content-Language: en-GB To: Catalin Marinas , Will Deacon Cc: =?UTF-8?Q?Miko=C5=82aj_Lenczewski?= , suzuki.poulose@arm.com, yang@os.amperecomputing.com, corbet@lwn.net, jean-philippe@linaro.org, robin.murphy@arm.com, joro@8bytes.org, akpm@linux-foundation.org, paulmck@kernel.org, mark.rutland@arm.com, joey.gouly@arm.com, maz@kernel.org, james.morse@arm.com, broonie@kernel.org, oliver.upton@linux.dev, baohua@kernel.org, david@redhat.com, ioworker0@gmail.com, jgg@ziepe.ca, nicolinc@nvidia.com, mshavit@google.com, jsnitsel@redhat.com, smostafa@google.com, kevin.tian@intel.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev References: <20250428153514.55772-2-miko.lenczewski@arm.com> <20250428153514.55772-4-miko.lenczewski@arm.com> <20250506142508.GB1197@willie-the-truck> <78fec33d-fe66-4352-be11-900f456c9af3@arm.com> <20250509134904.GA5707@willie-the-truck> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 09/05/2025 17:04, Catalin Marinas wrote: > On Fri, May 09, 2025 at 02:49:05PM +0100, Will Deacon wrote: >> On Tue, May 06, 2025 at 03:52:59PM +0100, Ryan Roberts wrote: >>> On 06/05/2025 15:25, Will Deacon wrote: >>>> This penalises large homogeneous systems and it feels unnecessary given >>>> that we have the ability to check this per-CPU. Can you use >>>> ARM64_CPUCAP_BOOT_CPU_FEATURE instead of ARM64_CPUCAP_SYSTEM_FEATURE >>>> to solve this? >>> >>> We are trying to solve for the case where the boot CPU has BBML2 but a secondary >>> CPU doesn't. (e.g. hetrogeneous system where boot CPU is big and secondary is >>> little and does not advertise the feature. I can't remember if we proved there >>> are real systems with this config - I have vague recollection that we did but my >>> memory is poor...). >>> >>> My understanding is that for ARM64_CPUCAP_BOOT_CPU_FEATURE, "If the boot CPU >>> has enabled this feature already, then every late CPU must have it". So that >>> would exclude any secondary CPUs without BBML2 from coming online? >> >> Damn, yes, you're right. However, it still feels horribly hacky to iterate >> over the online CPUs in has_bbml2_noabort() -- the cpufeature framework >> has the ability to query features locally and we should be able to use >> that. We're going to want that should the architecture eventually decide >> on something like BBML3 for this. >> >> What we have with BBML2_NOABORT seems similar to an hwcap in that we only >> support the capability if all CPUs have it (rejecting late CPUs without it >> in that case) but we can live without it if not all of the early CPUs >> have it. Unlikely hwcaps, though, we shouldn't be advertising this to >> userspace and we can't derive the capability solely from the sanitised >> system registers. >> >> I wonder if we could treat it like an erratum in some way instead? That >> is, invert things so that CPUs which _don't_ have BBML2_NOABORT are >> considered to have a "BBM_CONFLICT_ABORT" erratum (which we obviously >> wouldn't shout about). Then we should be able to say: >> >> - If any of the early CPUs don't have BBML2_NOABORT, then the erratum >> would be enabled and we wouln't elide BBM. >> >> - If a late CPU doesn't have BBML2_NOABORT then it can't come online >> if the erratum isn't already enabled. >> >> Does that work? If not, then perhaps the cpufeature/cpuerrata code needs >> some surgery for this. > > Ah, I should have read this thread in order. I think we can treat this > as BBML2_NOABORT available as default based on ID regs and use the > allow/deny-list as an erratum. > Just to make sure I've understood all this, I think what you are both saying is we can create a single capability called ARM64_HAS_NO_BBML2_NOABORT of type ARM64_CPUCAP_LOCAL_CPU_ERRATUM. Each CPU will then check it has BBML2 and is in the MIDR allow list; If any of those conditions are not met, the CPU is considered to have ARM64_HAS_NO_BBML2_NOABORT. Then we have this helper: static inline bool system_supports_bbml2_noabort(void) { return system_capabilities_finalized() && !alternative_has_cap_unlikely(ARM64_HAS_NO_BBML2_NOABORT); } system_capabilities_finalized() is there to ensure an early call to this helper returns false (i.e. the safe value before we have evaluated on all CPUs). Because ARM64_HAS_NO_BBML2_NOABORT is inverted it would otherwise return true prior to finalization. I don't believe we need any second (SYSTEM or BOOT) feature. This is sufficient on its own? Thanks, Ryan