linux-kernel - Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV

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Date: Mon, 10 Jun 2024 21:28:39 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.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 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl

On Mon, Jun 10, 2024 at 04:04:30PM -0700, Nicolin Chen wrote:

> > > Actually, even now as we put a dispatcher in VMM, VMM still does
> > > decode the CD table to link ASID to s1_hwpt. Otherwise, it could
> > > only broadcast a TLBI cmd to all pSMMUs.
> > 
> > No, there should be no CD table decoding and no linking ASID to
> > anything by the VMM.
> > 
> > The ARM architecture is clean, the ASID can remain private to the VM,
> > there is no reason for the VMM to understand it.
> 
> But a guest-level TLBI command usually has only ASID available to
> know which pSMMU to dispatch the command. Without an ASID lookup
> table, how could VMM then dispatch a command to the corresponding
> pSMMU?

It can broadcast. The ARM architecture does not expect a N:1 mapping
of SMMUs. This is why I think it is not such a good idea..

Yes the VMM could walk the CD tables too and build up a bitmap of what
ASIDs are being used by what pSMMUs, and that would be fine for the
VMM to do, but I wouldn't necessarily recommend it :)

Jason