linux-kernel - RE: [PATCH v1 02/14] iommufd: Add nesting related data structures for ARM SMMUv3

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Message-ID: <BN9PR11MB5276B843A5E24B8CF5F4449E8CBD9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Fri, 17 Mar 2023 10:04:59 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        "will@...nel.org" <will@...nel.org>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>
Subject: RE: [PATCH v1 02/14] iommufd: Add nesting related data structures for
 ARM SMMUv3

> From: Robin Murphy <robin.murphy@....com>
> Sent: Friday, March 10, 2023 11:57 PM
> 
> >
> > If iommufd could provide a general cross-driver API to set exactly
> > that scenario up then VMM code could also be general. That seems
> > prettty interesting.
> 
> Indeed, I've always assumed the niche for virtio would be that kind of
> in-between use-case using nesting to accelerate simple translation,
> where we plug a guest-owned pagetable into a host-owned context. That
> way the guest retains the simple virtio interface and only needs to
> understand a pagetable format (or as you say, simply share a CPU
> pagetable) without having to care about the nitty-gritty of all the
> IOMMU-specific moving parts around it. For guests that want to get into
> more advanced stuff like managing their own PASID tables, pushing them
> towards "native" nesting probably does make more sense.
> 

Interesting thing is that we cannot expose both virtio-iommu and
emulated vIOMMU to one guest to choose. then if the guest has
been using virtio-iommu for whatever reason naturally it may
want more advanced features on it too.