linux-kernel - Re: [PATCH v2 17/19] iommu/arm-smmu-v3: Add arm_smmu_viommu_cache

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Message-ID: <ZuIDPGue40n02kFE@nvidia.com>
Date: Wed, 11 Sep 2024 13:53:16 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
CC: Jason Gunthorpe <jgg@...dia.com>, "will@...nel.org" <will@...nel.org>,
	"joro@...tes.org" <joro@...tes.org>, "suravee.suthikulpanit@....com"
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	<shameerali.kolothum.thodi@...wei.com>, "smostafa@...gle.com"
	<smostafa@...gle.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: Re: [PATCH v2 17/19] iommu/arm-smmu-v3: Add
 arm_smmu_viommu_cache_invalidate

On Wed, Sep 11, 2024 at 08:13:01AM +0000, Tian, Kevin wrote:
> > > > Yes, many more patches, and don't try to do it now.. But we can copy
> > > > the vmid from the s2 and place it in the viommu struct during
> > > > allocation time.
> > > >
> > >
> > > does it assume that a viommu object cannot span multiple physical
> > > IOMMUs so there is only one vmid per viommu?
> >
> > I think so. One the reasons of introducing vIOMMU is to maintain
> > the shareability across physical IOMMUs at the s2 HWPT_PAGING.
> >
> 
> I don't quite get it. e.g. for intel-iommu the S2 domain itself can
> be shared across physical IOMMUs

SMMU does the same, but needs a VMID per pSMMU to tag that S2
domain:

vIOMMU0 (VMIDx of pSMMU0) -> shared S2
vIOMMU1 (VMIDy of pSMMU1) -> shared S2

Note: x and y might be different.

> then what is the problem
> preventing a vIOMMU object using that S2 to span multiple IOMMUs?

Jason previously suggested the way of implementing multi-vIOMMU
in a VMM to be one vIOMMU object representing a vIOMMU instance
(of a physical IOMMU) in the VM. So, it'd be only one VMID per
one vIOMMU object.

Sharing one vIOMMU object on the other hand needs the vIOMMU to
hold a list of VMIDs for all (or attached?) physical IOMMUs. This
would change what a vIOMMU object represents.

> Probably there is a good reason e.g. for simplification or better
> aligned with hw accel stuff. But it's not explained clearly so far.

I will try emphasizing that in the next version, likely in the
rst file that I am patching for HWPT_PAGING/NESTED at this point.

Thanks
Nicolin