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 6D09528D8FD for ; Fri, 9 May 2025 13:02:08 +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=1746795730; cv=none; b=S/CX25HDqWF2/l7MydyTHAG65in3BmiCIf1ckzjerH6Au9HV4W0zeH/LK9Ve4FvDy1sUqiExP/tCpo69Og0MIN1z2JPiV/oaDzhdfsS8E6LrQQrwhVNqOk9nA/WX+JxFc+oqvZeo2SQ51knYOSMUNEv/hG/yxKPMSoH+1GqlNFc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746795730; c=relaxed/simple; bh=Kze0SyEYvQc8SDRBVeNXW/G/mCAizeF6tVAMrvRPVhA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YQm4CjU1rWtteRs43vHoGZaCOIEfy/UPzYlFtIkfxAVUygfhmfZtG2F1Ka2TFVwsjwCjbQ08y2iZ+IzOyg4RKNHFnz5m1tFOHRdlBlB1NDHiOtFCXDAHC2E8aKWtJA+Ty6S9v0rJ+bnwO/27PReWEyxpTY2702rQo/ROlwhEOi4= 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 B18BA1595; Fri, 9 May 2025 06:01:56 -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 87F603F58B; Fri, 9 May 2025 06:02:05 -0700 (PDT) Message-ID: <2ad910f4-6930-4da2-aa2b-f3875f71e001@arm.com> Date: Fri, 9 May 2025 14:02:03 +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 v5 0/9] Merge arm64/riscv hugetlbfs contpte support Content-Language: en-GB To: Alexandre Ghiti , Will Deacon , Lorenzo Stoakes Cc: Alexandre Ghiti , Catalin Marinas , Mark Rutland , Matthew Wilcox , Paul Walmsley , Palmer Dabbelt , Andrew Morton , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, linux-mm@kvack.org References: <20250321130635.227011-1-alexghiti@rivosinc.com> <4dd5d187-f977-4f27-9937-8608991797b5@ghiti.fr> <64409a13-1c07-42cd-b1ec-572042738f1b@arm.com> <84cb893a-46e3-408a-ba0e-2eff0b44d2a1@ghiti.fr> <20250508123046.GA3706@willie-the-truck> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 09/05/2025 12:09, Alexandre Ghiti wrote: > Hi Will, > > On Thu, May 8, 2025 at 2:30 PM Will Deacon wrote: >> >> Hi folks, >> >> On Mon, May 05, 2025 at 06:08:50PM +0200, Alexandre Ghiti wrote: >>> On 29/04/2025 16:09, Ryan Roberts wrote: >>>> On 07/04/2025 13:04, Alexandre Ghiti wrote: >>>>> Can someone from arm64 review this? I think it's preferable to share the same >>>>> implementation between riscv and arm64. >>>> I've been thinking about this for a while and had some conversations internally. >>>> This patchset has both pros and cons. >>>> >>>> In the pros column, it increases code reuse in an area that has had quite of few >>>> bugs popping up lately; so this would bring more eyes and hopefully higher >>>> quality in the long run. >>>> >>>> But in the cons column, we have seen HW errata in similar areas in the past and >>>> I'm nervous that by hoisting this code to mm, we make it harder to workaround >>>> any future errata. Additionally I can imagine that this change could make it >>>> harder to support future Arm architecture enhancements. >>>> >>>> I appreciate the cons are not strong *technical* arguments but nevertheless they >>>> are winning out in this case; My opinion is that we should keep the arm64 >>>> implementations of huge_pte_ (and contpte_ too - I know you have a separate >>>> series for this) private to arm64. >>>> >>>> Sorry about that. >>>> >>>>> The end goal is the support of mTHP using svnapot on riscv, which we want soon, >>>>> so if that patchset does not gain any traction, I'll just copy/paste the arm64 >>>>> implementation into riscv. >>>> This copy/paste approach would be my preference. >>> >>> >>> I have to admit that I disagree with this approach, the riscv and arm64 >>> implementations are *exactly* the same so it sounds weird to duplicate code, >>> the pros you mention outweigh the cons. >>> >>> Unless I'm missing something about the erratas? To me, that's easily fixed >>> by providing arch specific overrides no? Can you describe what sort of >>> erratas would not fit then? One concrete feature is the use of Arm's FEAT_BBM level 2 to avoid having to do break-before-make and TLB maintenance when doing a fold or unfold operation. There is a series in flight to add this support at [1]. I can see this type of approach being extended to the hugetlb helpers in future. I also have another series in flight at [2] that tidies up the hugetlb implementation and does some optimizations. But the optimizations depend on arm64-specific TLB maintenance APIs. [1] https://lore.kernel.org/linux-arm-kernel/20250428153514.55772-2-miko.lenczewski@arm.com/ [2] https://lore.kernel.org/linux-arm-kernel/20250422081822.1836315-1-ryan.roberts@arm.com/ As for errata, that's obviously much more fuzzy; there have been a bunch relating to the MMU in the recent past, and I wouldn't be shocked if more turned up. For future architecture enchancements, I'm aware of one potential feature being discussed for which this change would likely make it harder to implement. >> >> If we start with the common implementation you have here, nothing >> prevents us from forking the code in future if the architectures diverge >> so I'd be inclined to merge this series and see how we get on. OK if that's your preference, I'm ok with it. I don't have strong opinion, just a sense that we will end up with loads of arch-specific overrides. As you say, let's see. Alexandre, I guess this series is quite old now and will need to incorporate the hugtelb fixes I did last cycle? And ideally I'd like [2] to land then for that to also be incorporated into your next version. (I'm still hopeful we can get [2] into v6.16 and have been waiting patiently for Will to pick it up ;) ). I guess we can worry about [1] later as that is only affected by your other series. How does that sound? >> However, >> one thing I *do* think we need to ensure is that the relevant folks from >> both arm64 (i.e. Ryan) and riscv (i.e. Alexandre) are cc'd on changes to >> the common code. Otherwise, it's going to be a step backwards in terms >> of maintainability. >>>> Could we add something to MAINTAINERS so that the new file picks you both >> up as reviewers? That's fine with me. Lorenzo added me for some parts of MM this cycle anyway. Thanks, Ryan > > I'm adding Lorenzo as he is cleaning the mm MAINTAINERS entries. > > @Lorenzo: should we add a new section "CONTPTE" for this? FYI, hugetlb > is the first patchset, I have another patchset to merge THP contpte > support [1] as well so the "HUGETLB" section does not seem to be a > good fit. > > [1] https://lore.kernel.org/linux-riscv/20240508191931.46060-1-alexghiti@rivosinc.com/ > > Thanks, > > Alex > >> >> Will