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

[Pranjal Shrivastava <praan@google.com>]

On Tue, Oct 15, 2024 at 03:00:10PM +0000, Pranjal Shrivastava wrote:
> On Tue, Oct 15, 2024 at 09:47:23AM -0300, 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.
> > 
> > If this is a common thing we could have the core code take on more of
> > the job.
> 
> Yes! I've seen the IOMMU being bypassed at multiple places, primarily
> for performance, people like bypassing the iommu for "trusted" devices.
> A few examples that are publically accessible: Qcomm SoCs [1], [2].
> Seems like Qualcomm have a DT property `qcomm-s1-bypass` to achieve
> something similar.
> 
> In fact, *blast from the past*, I tried to do something similar sometime
> ago with [3]. Although, perhaps that wasn't the best way (and I was a
> kernel newbie :))
> 
> A little off-topic, but I think there has been some interest to bypass
> the default substream as well while still maintaining PASID isolation.[4]
> 

Agh, apologies, ignore the following part about rmr,
it won't solve the problem here.
> Although, as far as arm-smmu-v3 is concerned, IIRC, I think there was a
> way to tell that the region is reserved and don't map it.

> 
> > 
> > Jason
> 
> Thanks,
> Pranjal
> 
> [1]
> https://github.com/realme-kernel-opensource/realme5-kernel-source/blob/master/arch/arm64/boot/dts/qcom/sa8155-vm-qupv3.dtsi#L22
> 
> [2] 
> https://android.googlesource.com/kernel/msm/+/android-7.1.0_r0.2/Documentation/devicetree/bindings/platform/msm/ipa.txt#28
> 
> [3]
> https://lore.kernel.org/all/20230707104857.348353-1-praan@google.com/
> 
> [4]
> https://lore.kernel.org/all/CAGfWUPziSWNMc_px4E-i+_V_Jxdb_WSwOLXHZ+PANz2Tv5pFPA@mail.gmail.com/