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 4E538364953; Wed, 24 Jun 2026 11:42:23 +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=1782301345; cv=none; b=fmZ/HDHkZaooaK51DKDVwTmzjOnr566iygv9anZCo3GHdLcw3IlyM5WVHgNICdnQp6vcI5VLvtQzxAfRrTA6iN+QcF6nniCPsOc6JHVotB407r99X+CT1WqrcbOoSMmaf8bN3XsXufWcD5iWZzFk1sQhUJsbVFxu7LZwJMyUXC8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782301345; c=relaxed/simple; bh=riMPr912UFIPMnkd77dj4O/2qenW/y+bhggkPiDqP2Y=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uFu3olmiXFaChTHCJr3xmY5hgt1RyqrC0uyN8+Sq4XhHKKl3Ee4ya4hIdSMwPSmtHgl11e3ACAjVpYsjNND2lF2PwRo4yNfC+pRPAuobLnBZa9gT6/ZCqtG35fMJl6SpMCdZbfyCSH1FxuiWVwkn3w1hCsRw/cgNYBFUkp1mg+o= 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=Vfw1P4/4; 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="Vfw1P4/4" 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 9855215A1; Wed, 24 Jun 2026 04:42:17 -0700 (PDT) Received: from [10.2.212.23] (e121345-lin.cambridge.arm.com [10.2.212.23]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 263A93F632; Wed, 24 Jun 2026 04:42:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1782301342; bh=riMPr912UFIPMnkd77dj4O/2qenW/y+bhggkPiDqP2Y=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Vfw1P4/4IoI5cqXw4yBJZNOTaVkYRmMS6+C6sbeyrmmGnGWWzzWff8oMzVCkvRHpG /WfA6cZgcjrIZBg4la2twn3GyGmuyUH3yH5HcauiNCm7UhB5ttO+fuGOsQ/50nUI2x XbN64JsbCSXXQ+WSm4c6Fyy4/VKP2cHszV2Zyuew= Message-ID: Date: Wed, 24 Jun 2026 12:42:17 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/9] PCI/P2PDMA: Add CONFIG_PCI_P2PDMA_CORE To: Matt Evans , "Tian, Kevin" , Pranjal Shrivastava Cc: Alex Williamson , Leon Romanovsky , Jason Gunthorpe , Alex Mastro , =?UTF-8?Q?Christian_K=C3=B6nig?= , Bjorn Helgaas , Logan Gunthorpe , Mahmoud Adam , David Matlack , =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , Sumit Semwal , Ankit Agrawal , Alistair Popple , "Kasireddy, Vivek" , "linux-kernel@vger.kernel.org" , "linux-media@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , "linaro-mm-sig@lists.linaro.org" , "kvm@vger.kernel.org" , "linux-pci@vger.kernel.org" References: <20260610154327.37758-1-matt@ozlabs.org> <20260610154327.37758-2-matt@ozlabs.org> <0dfadf98-a904-4e6a-b078-5caf27bc7922@ozlabs.org> <81a9d212-996e-42e1-aabf-b120a13e94c3@arm.com> <88a9ebaa-8637-4290-b299-acae0e3065f8@ozlabs.org> From: Robin Murphy Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 23/06/2026 6:47 pm, Matt Evans wrote: > Hi Robin, me, > > On 23/06/2026 16:59, Matt Evans wrote: >> Heya Robin, >> >> On 23/06/2026 16:48, Robin Murphy wrote: >>> On 12/06/2026 3:31 pm, Matt Evans wrote: >>>> Hi Kevin, Pranjal, (+Robin, hi!) >>> >>> Oh hey there! :) >>> >>>> On 12/06/2026 04:39, Tian, Kevin wrote: >>>>>> From: Pranjal Shrivastava >>>>>> Sent: Friday, June 12, 2026 2:38 AM >>>>>> >>>>>> On Wed, Jun 10, 2026 at 04:43:15PM +0100, Matt Evans wrote: >>>>>>> --- a/drivers/pci/Kconfig >>>>>>> +++ b/drivers/pci/Kconfig >>>>>>> @@ -206,11 +206,7 @@ config PCIE_TPH >>>>>>>   config PCI_P2PDMA >>>>>>>       bool "PCI peer-to-peer transfer support" >>>>>>>       depends on ZONE_DEVICE >>>>>>> -    # >>>>>>> -    # The need for the scatterlist DMA bus address flag means PCI >>>>>> P2PDMA >>>>>>> -    # requires 64bit >>>>>>> -    # >>>>>>> -    depends on 64BIT >>>>>>> +    select PCI_P2PDMA_CORE >>>>>>>       select GENERIC_ALLOCATOR >>>>>>>       select NEED_SG_DMA_FLAGS >>>>>>>       help >>>>>> >>>>>> Nit: Did we drop depends on 64BIT intentionally here? I guess the full >>>>>> PCI_P2PDMA stack still selects NEED_SG_DMA_FLAGS? IIRC, >>>>>> NEED_SG_DMA_FLAGS doesn't select 64BIT? >>>>> >>>>> seems that comment is stale. According to the commit msg: >>>>> >>>>> " it would make vfio-pci only available if CONFIG_ZONE_DEVICE is >>>>> present (e.g. 64-bit systems), " >>>>> >>>>> so it sounds a redundant dependency hence is removed. >>>> >>>> This was intentional.  In practice there is still a dependency on 64BIT >>>> for PCI_P2PDMA, but it is because of ZONE_DEVICE (and mem hotplug).  The >>>> key need is PCI_P2PDMA_CORE is available on !64BIT for VFIO, but I >>>> didn't see a requirement from PCI_P2PDMA itself (as opposed to its >>>> dependencies).  If I've missed one, I can put it back... >>>> >>>> But NEED_SG_DMA_FLAGS doesn't smell quite right; I see from comments in >>>> >>>>    af2880ec44021 ("scatterlist: add dedicated config for DMA flags") >>>> >>>> that it assumes 64BIT, but it seems to be missing a "depends on 64BIT". >>>> >>>> Robin -- should that depend on 64BIT? >>> >>> Indeed, looking at the history it seems like that was overlooked, but it >>> worked out at the time since the only selector of NEED_SG_DMA_FLAGS was >>> PCI_P2PDMA as you say. If we're now generalising then moving the >>> explicit 64BIT dependency to NEED_SG_DMA_FLAGS itself sounds like the >>> right thing to do. >> >> Cheers for confirming. I'll send a patch separate to this series (since >> the deps work out OK for PCI_P2PDMA for the reasons mentioned). > > I think we were wrong, NEED_SG_DMA_FLAGS doesn't _need_ 64BIT. > > Other than P2PDMA, the other consumer of NEED_SG_DMA_FLAGS is IOMMU_DMA, > and turns out if one builds an i386 kernel with INTEL_IOMMU (or some > other configs, like Xen) then NEED_SG_DMA_FLAGS is enabled on 32-bit > builds too. > > The scatterlist.h comments af2880ec44021 touched are just saying that > _since_ P2PDMA depends on 64BIT, there _is_ circumstantial padding so > let's use it for flags. It doesn't require 64BIT. > > For example struct scatterlist isn't pushed over some special (e.g. > power-of-two) size when NEED_SG_DMA_FLAGS is enabled on 32-bit; I can't > find a reason it should be prevented on 32-bit builds (and found cases > above in which it is already enabled in them). > > So I won't change the NEED_SG_DMA_FLAGS dependencies after all. Sorry > for the noise -- as ever if I've missed something do please explain. Ah, I was also taking the comments at face value, plus conflating the memory of the original series which did stash the P2P flag in bit 2 of the page_link pointer - in my defence, it's far too hot for my brain to work properly :) Doing a bit more archaeology, the separate dma_flags field was a late suggestion just before the series was merged, so I guess it's actually just that the comments never got updated accordingly. Sorry for perpetuating the confusion! Cheers, Robin. > (I'll continue with removing the P2PDMA dependency on 64BIT because it > seems P2PDMA's dependencies rely on 64BIT, though P2PDMA itself doesn't.) > > Thanks, > > > Matt