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 EC064355036; Tue, 27 Jan 2026 13:26:06 +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=1769520368; cv=none; b=OM6cFMXYJTgcxdkmFsBcvCXyYDw+XmgukYnRcsK5HQA2EW2alUwhtxQS42mzya1OM8fjU7jCYJxpS3prBnI3d6JC6Pt1QGpdW6VGrMFv1hbRstsSnr/ZoAXfOeNEPYrfGWS3wizLNytizrgKujzGjO2BYWWDar9zdDwK5e9a/+E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769520368; c=relaxed/simple; bh=dHLLCoOksVEiX5+cGAAgXnOR7Mo3CHmYyJCj6LhfSv4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=FgzEJeEp/vOHW1WQTgUdwY6vqY5a784DpBtBKfc+dL2eTbYADJ+LGnhvrGTaRMlAnnJTKyi+SDGvn8w6LuWIAl+NDTT7Gxdwj9PVhXIPlj/IT2fpmM/3RN6zISjHiKqlXD/JzijKa5CwFmuYhJHCaFoqsQ+zUTpZxrTRbFjNgy8= 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 E6B9D1595; Tue, 27 Jan 2026 05:25:59 -0800 (PST) Received: from [10.57.51.176] (unknown [10.57.51.176]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3BF3C3F632; Tue, 27 Jan 2026 05:26:04 -0800 (PST) Message-ID: Date: Tue, 27 Jan 2026 13:26:02 +0000 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 RFCv1 3/3] iommu/arm-smmu-v3: Allow ATS to be always on To: Will Deacon , Jason Gunthorpe Cc: Nicolin Chen , bhelgaas@google.com, joro@8bytes.org, praan@google.com, baolu.lu@linux.intel.com, kevin.tian@intel.com, miko.lenczewski@arm.com, linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org References: <09cb6be1f8f7472a2f1ccab72154cc6e22cf570b.1768624181.git.nicolinc@nvidia.com> <20260126172020.GP1134360@nvidia.com> <0a60797a-fbb9-4a83-adc6-ad62baf5915b@arm.com> <20260126190935.GV1134360@nvidia.com> From: Robin Murphy Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2026-01-27 1:10 pm, Will Deacon wrote: > On Mon, Jan 26, 2026 at 03:09:35PM -0400, Jason Gunthorpe wrote: >> On Mon, Jan 26, 2026 at 06:49:07PM +0000, Robin Murphy wrote: >>> (assuming SSIDSIZE > 0 and it does anything at all - note that strictly we >>> cannot assume this bypass trick is *always* possible, since an SMMU is >>> permitted to support ATS without supporting SubStreams). >> >> Yes, I think Nicolin has captured those conditions in computing >> it... We don't have a logic to disable bypass in that case though. >> >>>> So, I think a CD table pointer to a fully invalid L1 table of at least >>>> size 1 should be OK? >>>> >>>> Or stated another way, why would ie be OK to have a 1 level table with >>>> an non-valid CD table entry for SSID0 but not OK to have a 2 level >>>> table that returns non-valid at the first walk? >>> >>> S1ContextPtr itself is reachable since S1 is enabled, so it cannot point to >>> nonsense. But the S1DSS==Bypass behaviour does state: >> >>> "Note: Such a transaction does not fetch a CD, and therefore does not report >>> F_CD_FETCH, C_BAD_CD or a stage 2 Translation-related fault with CLASS == >>> CD." >> >> Yes >> >> However, taken together: >> * S1CDMax is set to substream 0 only >> * S1DSS is set such that "does not fetch a CD" for SSID = 0 >> * SSID >0 doesn't fetch CDs because of S1CDMax >> >> Then it seems to be saying that it will never use S1ContextPtr? ie it >> is IGNORED? > > Right, I think the critical question is whether that setting of S1DSS > (0b01) means that STE.S1ContextPtr is considered "invalid". The spec > doesn't call this out explicitly but the "translation procedure charts" > seem to indicate that it doesn't use the CD for anything... > > It would be good to get some clarification from Arm about this > particular case. No, STE.S1ContextPtr itself is "valid" since S1 is enabled. No CD fetch will occur for no-SubStreamID transactions that are bypassed by S1DSS, but the SMMU is permitted to attempt to speculatively fetch CDs for the enabled SubStreamID(s). Those fetches do not have to reach a valid CD if the SubStream is not actually in use, much like we don't have to fully populate a 2-level Stream table for StreamID ranges we don't care about either. Don't confuse S1DSS==1 (bypass) with the S1DSS==2 behaviour we use in other cases - the latter is "Use CD 0 for no-SubstreamID traffic" which makes SubStreamID 0 invalid to use. However in the bypass case (and also S1DSS==0 where no-SubstreamID traffic is blocked entirely), SubStreamID 0 remains perfectly valid and usable (we just still won't ever use it in Linux due to the middle case). Thanks, Robin.