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

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Message-ID: <BN9PR11MB5276EBE0F714CF3EF0DAEDEC8C9B2@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 11 Sep 2024 08:13:01 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Nicolin Chen <nicolinc@...dia.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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	<shuah@...nel.org>, "linux-kernel@...r.kernel.org"
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	<iommu@...ts.linux.dev>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kselftest@...r.kernel.org"
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	<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

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Wednesday, September 11, 2024 3:21 PM
> 
> On Wed, Sep 11, 2024 at 06:25:16AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Friday, September 6, 2024 2:22 AM
> > >
> > > On Thu, Sep 05, 2024 at 11:00:49AM -0700, Nicolin Chen wrote:
> > > > On Thu, Sep 05, 2024 at 01:20:39PM -0300, Jason Gunthorpe wrote:
> > > > > On Tue, Aug 27, 2024 at 09:59:54AM -0700, Nicolin Chen wrote:
> > > > >
> > > > > > +static int arm_smmu_viommu_cache_invalidate(struct
> > > iommufd_viommu *viommu,
> > > > > > +                                           struct iommu_user_data_array
> > > *array)
> > > > > > +{
> > > > > > +       struct iommu_domain *domain =
> > > iommufd_viommu_to_parent_domain(viommu);
> > > > > > +
> > > > > > +       return __arm_smmu_cache_invalidate_user(
> > > > > > +                       to_smmu_domain(domain), viommu, array);
> > > > >
> > > > > I'd like to have the viommu struct directly hold the VMID. The nested
> > > > > parent should be sharable between multiple viommus, it doesn't
> make
> > > > > any sense that it would hold the vmid.
> > > > >
> > > > > This is struggling because it is trying too hard to not have the
> > > > > driver allocate the viommu, and I think we should just go ahead and
> do
> > > > > that. Store the vmid, today copied from the nesting parent in the vmid
> > > > > private struct. No need for iommufd_viommu_to_parent_domain(),
> just
> > > > > rework the APIs to pass the vmid down not a domain.
> > > >
> > > > OK. When I designed all this stuff, we still haven't made mind
> > > > about sharing the s2 domain, i.e. moving the VMID, which might
> > > > need a couple of more patches to achieve.
> > >
> > > 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 then what is the problem 
preventing a vIOMMU object using that S2 to span multiple IOMMUs?

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.