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

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Message-ID: <Zw-DZF4B4psiat59@google.com>
Date: Wed, 16 Oct 2024 09:12:04 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Peng Fan <peng.fan@....com>
Cc: Robin Murphy <robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>,
	"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" <linux-arm-kernel@...ts.infradead.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 2/2] iommu/arm-smmu-v3: Bypass SID0 for NXP i.MX95

On Wed, Oct 16, 2024 at 09:02:39AM +0000, Peng Fan wrote:
> All,
> 
> > Subject: Re: [PATCH RFC 2/2] iommu/arm-smmu-v3: Bypass SID0 for
> > NXP i.MX95
> 
> Thanks for the discussion on this topic to show much information
> that I not foresee.
> 
> > 
> > On Tue, Oct 15, 2024 at 04:37:25PM +0100, Robin Murphy wrote:
> > > 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.
> > 
> > Agreed. I assumed the bypass was needed for a registered SID.
> 
> I just reply here, not reply each thread.

Apologies, I responded to the other thread before looking at this one

> 
> The SID is not a registered SID.
> 
> Considering the security things, except "iommus = <&smmu 0>"
> being added, is there other method for this issue?

I can only think of RMRs if you can ensure/restrict eDMA3 to access a
fixed region of memory. Something like a DMA zone if feasible.

> 
> Thanks,
> Peng.
> 
> > 
> > >
> > > Thanks,
> > > Robin.
> > 
Thanks,
Pranjal