From:   Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
CC:     Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Nicolin Chen <nicolinc@nvidia.com>,
        "robin.murphy@arm.com" <robin.murphy@arm.com>,
        "will@kernel.org" <will@kernel.org>,
        "eric.auger@redhat.com" <eric.auger@redhat.com>,
        "kevin.tian@intel.com" <kevin.tian@intel.com>,
        "baolu.lu@linux.intel.com" <baolu.lu@linux.intel.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "iommu@lists.linux.dev" <iommu@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "yi.l.liu@intel.com" <yi.l.liu@intel.com>
Subject: RE: [PATCH v1 02/14] iommufd: Add nesting related data structures for
 ARM SMMUv3
Thread-Topic: [PATCH v1 02/14] iommufd: Add nesting related data structures
 for ARM SMMUv3
Thread-Index: AQHZUnWTVXhuXoh0OkWPSyQY1srSfq7ydQKAgAAVzLCAAAsqAIAAANyQ
Date:   Thu, 9 Mar 2023 15:51:42 +0000
Message-ID: <39979642ba8042ba9d4de651fecfeffb@huawei.com>
References: <cover.1678348754.git.nicolinc@nvidia.com>
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 <20230309134217.GA1673607@myrica>
 <213a05ef06944a55a148995053b187f8@huawei.com> <ZAn94BXkjiJKQ66O@nvidia.com>
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> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgg@nvidia.com]
> Sent: 09 March 2023 15:40
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>; Nicolin Chen
> <nicolinc@nvidia.com>; robin.murphy@arm.com; will@kernel.org;
> eric.auger@redhat.com; kevin.tian@intel.com; baolu.lu@linux.intel.com;
> joro@8bytes.org; linux-arm-kernel@lists.infradead.org;
> iommu@lists.linux.dev; linux-kernel@vger.kernel.org; yi.l.liu@intel.com
> Subject: Re: [PATCH v1 02/14] iommufd: Add nesting related data structures
> for ARM SMMUv3
> 
> On Thu, Mar 09, 2023 at 03:26:12PM +0000, Shameerali Kolothum Thodi
> wrote:
> 
> > On platforms that supports BTM [1], we may need the VMID allocated by
> KVM.
> > But again getting that from user pace doesn't look safe. I have attempted
> to revise
> > the earlier RFC to pin and use the KVM VMID from SMMUv3 here[2].
> 
> Gurk
> 
> > " In the new design we can require from the start that creating a nesting
> IOMMU
> > container through /dev/iommu *must* come with a KVM context, that way
> > we're sure to reuse the existing VMID. "
> 
> I've been dreading this but yes I execpt we will eventually need to
> connect kvm and iommufd together. The iommu driver can receive a kvm
> pointer as part of the alloc domain operation to do any setups like
> this.

That will make life easier :)
 
> If there is no KVM it should either fail to setup the domain or setup
> a domain disconnected from KVM.
> 

If no KVM the SMMUv3 can fall back to its internal VMID allocation I guess.
And my intention was to use KVM VMID only if the platform supports
BTM.

> If IOMMU HW and KVM HW are using the same ID number space then
> arguably the two kernel drivers need to use a shared ID allocator in
> the arch, regardless of what iommufd/etc does. Using KVM should not be
> mandatory for iommufd.
> 
> 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. 

Thanks,
Shameer

> Nicolin, that seems to be missing in this series? I'm not entirely
> sure how to elegantly code it :\
> 
> Jason