linux-kernel - Re: [PATCH RFC 2/2] iommu/arm-smmu-v3: Bypass SID0 for NXP i.MX95

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Message-ID: <ce7cc76f-426f-4d19-b4be-3964647a2f2d@arm.com>
Date: Tue, 15 Oct 2024 16:37:25 +0100
From: Robin Murphy <robin.murphy@....com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Pranjal Shrivastava <praan@...gle.com>,
 "Peng Fan (OSS)" <peng.fan@....nxp.com>, Will Deacon <will@...nel.org>,
 Joerg Roedel <joro@...tes.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Joy Zou <joy.zou@....com>,
 linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 Peng Fan <peng.fan@....com>
Subject: Re: [PATCH RFC 2/2] iommu/arm-smmu-v3: Bypass SID0 for NXP i.MX95

On 2024-10-15 4:31 pm, Jason Gunthorpe wrote:
> On Tue, Oct 15, 2024 at 04:13:13PM +0100, Robin Murphy wrote:
>> On 2024-10-15 1:47 pm, Jason Gunthorpe wrote:
>>> On Tue, Oct 15, 2024 at 08:13:28AM +0000, Pranjal Shrivastava wrote:
>>>
>>>> Umm.. this was specific for rmr not a generic thing. I'd suggest to
>>>> avoid meddling with the STEs directly for acheiving bypass. Playing
>>>> with the iommu domain type could be neater. Perhaps, modify the
>>>> ops->def_domain_type to return an appropriate domain?
>>>
>>> Yeah, that is the expected way, to force the def_domain_type to
>>> IDENTITY and refuse to attach a PAGING/BLOCKED domain.
>>
>> There is no domain, this is bypassing an arbitrary StreamID not associated
>> with any device.
> 
> If the stream ID is going to flow traffic shouldn't it have a DT node
> for it? Something must be driving the DMA on this SID, and the kernel
> does need to know what that is in some way, even for basic security
> things like making sure VFIO doesn't get a hold of it :\

Exactly, hence this RFC is definitely not the right approach.

Thanks,
Robin.