linux-kernel - RE: [PATCH RFC v3 08/12] iommu/sva: Use attach/detach_pasid

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Message-ID: <BN9PR11MB5276BE861CF22DDC11D9D8038CED9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Tue, 12 Apr 2022 23:36:55 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>
CC:     Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC v3 08/12] iommu/sva: Use attach/detach_pasid_dev in
 SVA interfaces

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Tuesday, April 12, 2022 8:53 PM
> 
> >
> >> +	if (!handle) {
> >> +		ret = -ENOMEM;
> >> +		goto out_put_ioas;
> >> +	}
> >> +
> >> +	/* The reference to ioas will be kept until domain free. */
> >> +	domain = iommu_sva_alloc_domain(dev, ioas);
> >
> > Shouldn't we first try whether existing domains are compatible to this
> > device?
> 
> If we think that here domain represents a hardware pagetable actually
> used by IOMMU for a {device, pasid}, we are able to use per-{device,
> pasid} domain without checking compatibility. Sharing a domain among
> devices under the same IOMMU may be an optimization. That could be done
> in the IOMMU driver just like what vt-d driver is doing for pass-through
> DMA domains.
> 

there is only one hardware page table per mm in this case. Multiple domains
are required only due to compatibility reason as Jason/Robin pointed out
in SMMU case. Given all other places create multiple domains per ioas only
upon incompatibility, probably it's more consistent to doing so in this path
too...