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

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Date:   Mon, 11 Dec 2023 12:11:25 -0800
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...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 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 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?

Thanks
Nicolin