linux-kernel - RE: [PATCH RFCv1 04/14] iommufd: Add struct iommufd_viommu and iommufd_viommu

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Message-ID: <BN9PR11MB5276461630564C74C9369CB88CF52@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 24 May 2024 02:16:34 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Nicolin Chen <nicolinc@...dia.com>, "will@...nel.org" <will@...nel.org>,
	"robin.murphy@....com" <robin.murphy@....com>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	"joro@...tes.org" <joro@...tes.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-tegra@...r.kernel.org"
	<linux-tegra@...r.kernel.org>, "Liu, Yi L" <yi.l.liu@...el.com>,
	"eric.auger@...hat.com" <eric.auger@...hat.com>, "vasant.hegde@....com"
	<vasant.hegde@....com>, "jon.grimm@....com" <jon.grimm@....com>,
	"santosh.shukla@....com" <santosh.shukla@....com>, "Dhaval.Giani@....com"
	<Dhaval.Giani@....com>, "shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.com>
Subject: RE: [PATCH RFCv1 04/14] iommufd: Add struct iommufd_viommu and
 iommufd_viommu_ops

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, May 23, 2024 8:59 PM
> On Thu, May 23, 2024 at 01:43:45AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, May 22, 2024 9:39 PM
> > >
> > > The driver will then know it should program three different VMIDs for
> > > the same S2 page table, which matches the ARM expectation for
> > > VMID. That is to say we'd pass in the viommu as the pt_id for the
> > > iommu_hwpt_alloc. The viommu would imply both the S2 page table and
> > > any meta information like VMID the driver needs.
> >
> > Can you elaborate the aspect about "three different VMIDs"?
> 
> In SMMUv3 the cache is tagged by (VMID,ASID) where ASID is completely
> controlled by the guest.
> 
> Every time the guest observes a SMMUv3 instance it is allowed to
> creates its own private ASID number space for that instance. The guest
> could re-use ASID #1 on two instances.
> 
> So every SMMUv3 instance plugged into the guest needs to have its own
> unique VMID so that the overlapping ASID's are disambiguate. The above
> would create a VM where:
> 
>  deva -> vSMMUv3 #1
>  devb -> vSMMUv3 #2
>  devc -> No IOMMU

This assumes that each vSMMUv3 instance has only one ASID space
i.e. the guest cannot create multiple VMID's itself? 

> > > Hmm, given we currently have no known hardware entanglement
> between
> > > PRI and VIOMMU it does seem OK for PRI to just exist seperate for
> >
> > Isn't AMD vPPRLog for directly sending PRI request into the guest?
> 
> I think it is, but that would be a vQUEUE on the VIOMMU not adding a
> VIOMMU to Lu's patches, which is what I ment.
> 
> > > now. If someone needs them linked someday we can add a viommu_id to
> > > the create pri queue command.
> >
> > I'm more worried about the potential conflict between the vqueue
> > object here and the fault queue object in Baolu's series, if we want
> > to introduce vIOMMU concept to platforms which lack of the hw
> > support.
> 
> I assume the vPPRLog will steal all the PRI before it reaches the
> kernel, so once this is turned on Lu's path won't see anything.
> 

Okay, then we expect this vqueue object only for HW acceleration
while software-based fault logging is still routed via Baolu's work.