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

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Date:   Thu, 9 Mar 2023 16:07:54 +0000
From:   Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "will@...nel.org" <will@...nel.org>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "kevin.tian@...el.com" <kevin.tian@...el.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "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>,
        "yi.l.liu@...el.com" <yi.l.liu@...el.com>
Subject: RE: [PATCH v1 02/14] iommufd: Add nesting related data structures for
 ARM SMMUv3



> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgg@...dia.com]
> Sent: 09 March 2023 16:00
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
> Cc: Jean-Philippe Brucker <jean-philippe@...aro.org>; Nicolin Chen
> <nicolinc@...dia.com>; robin.murphy@....com; will@...nel.org;
> eric.auger@...hat.com; kevin.tian@...el.com; baolu.lu@...ux.intel.com;
> joro@...tes.org; linux-arm-kernel@...ts.infradead.org;
> iommu@...ts.linux.dev; linux-kernel@...r.kernel.org; yi.l.liu@...el.com
> Subject: Re: [PATCH v1 02/14] iommufd: Add nesting related data structures
> for ARM SMMUv3
> 
> On Thu, Mar 09, 2023 at 03:51:42PM +0000, Shameerali Kolothum Thodi
> wrote:
> 
> > > For ARM cases where there is no shared VMID space with KVM, the ARM
> > > VMID should be somehow assigned to the iommfd_ctx itself and the alloc
> > > domain op should receive it from there.
> >
> > Is there any use of VMID outside SMMUv3? I was thinking if nested domain
> alloc
> > doesn't provide the KVM instance, then SMMUv3 can use its internal VMID.
> 
> When we talk about exposing an SMMUv3 IOMMU CMDQ directly to
> userspace then
> VMID is the security token that protects it.
> 
> So in that environment every domain under the same iommufd should
> share the same VMID so that the CMDQ's also share the same VMID.
> 
> I expect this to be a common sort of requirement as we will see
> userspace command queues in the other HW as well.
> 
> So, I suppose the answer for now is that ARM SMMUv3 should just
> allocate one VMID per iommu_domain and there should be no VMID in the
> uapi at all.
> 
> Moving all iommu_domains to share the same VMID is a future patch.
> 
> Though.. I have no idea how vVMID is handled in the SMMUv3
> architecture. I suppose the guest IOMMU HW caps are set in a way that
> it knows it does not have VMID?

I think, Guest only sets up the SMMUv3 S1 stage and it doesn't use VMID.

Thanks,
Shameer