linux-kernel - Re: [PATCH v6 0/6] iommufd: Add nesting infrastructure (part 2/2)

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Message-ID: <20231211214846.GA3014157@nvidia.com>
Date:   Mon, 11 Dec 2023 17:48:46 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     Yi Liu <yi.l.liu@...el.com>,
        "Giani, Dhaval" <Dhaval.Giani@....com>,
        Vasant Hegde <vasant.hegde@....com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        joro@...tes.org, alex.williamson@...hat.com, kevin.tian@...el.com,
        robin.murphy@....com, baolu.lu@...ux.intel.com, cohuck@...hat.com,
        eric.auger@...hat.com, kvm@...r.kernel.org, mjrosato@...ux.ibm.com,
        chao.p.peng@...ux.intel.com, yi.y.sun@...ux.intel.com,
        peterx@...hat.com, jasowang@...hat.com,
        shameerali.kolothum.thodi@...wei.com, lulu@...hat.com,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, zhenzhong.duan@...el.com,
        joao.m.martins@...cle.com, xin.zeng@...el.com, yan.y.zhao@...el.com
Subject: Re: [PATCH v6 0/6] iommufd: Add nesting infrastructure (part 2/2)

On Mon, Dec 11, 2023 at 12:11:25PM -0800, Nicolin Chen wrote:
> On Mon, Dec 11, 2023 at 09:20:41AM -0400, Jason Gunthorpe wrote:
> > On Mon, Dec 11, 2023 at 08:35:09PM +0800, Yi Liu wrote:
> > > > So.. In short.. Invalidation is a PITA. The idea is the same but
> > > > annoying little details interfere with actually having a compltely
> > > > common API here. IMHO the uAPI in this series is fine. It will support
> > > > Intel invalidation and non-ATC invalidation on AMD/ARM. It should be
> > > > setup to allow that the target domain object can be any HWPT.
> > > 
> > > This HWPT is still nested domain. Is it? But it can represent a guest I/O
> > > page table (VT-d), guest CD table (ARM), guest CR3 Table (AMD, it seems to
> > > be a set of guest CR3 table pointers). May ARM and AMD guys keep me honest
> > > here.
> > 
> > I was thinking ARM would not want to use a nested domain because
> > really the invalidation is global to the entire nesting parent.
> > 
> > But, there is an issue with that - the nesting parent could be
> > attached to multiple iommu instances but we only want to invalidate a
> > single instance. 
> 
> I am still not sure about attaching an S2 domain to multiple
> SMMUs. An S2 domain is created per SMMU, and we have such a
> rejection in arm_smmu_attach_dev():
> 	} else if (smmu_domain->smmu != smmu)
> 		ret = -EINVAL;

I intend to remove that eventually

> I understand that it would be probably ideal to share the S2
> iopt among the SMMUs. But in the driver the objects (domain)
> holding a shared S2 iopt must be different to allocate their
> own VMIDs, right?

No, the vmid will be moved into the struct arm_smmu_master_domain

Jason