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

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Message-ID: <20240524132023.GU20229@nvidia.com>
Date: Fri, 24 May 2024 10:20:23 -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 Thu, May 23, 2024 at 10:40:07PM -0700, Nicolin Chen wrote:
> On Thu, May 23, 2024 at 06:42:56AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen <nicolinc@...dia.com>
> > > Sent: Saturday, April 13, 2024 11:47 AM
> > >
> > > Introduce a new ioctl to set a per-viommu device virtual id that should be
> > > linked to the physical device id (or just a struct device pointer).
> > >
> > > Since a viommu (user space IOMMU instance) can have multiple devices
> > 
> > this is true...
> > 
> > > while
> > > it's not ideal to confine a device to one single user space IOMMU instance
> > > either, these two shouldn't just do a 1:1 mapping. Add two xarrays in
> > 
> > ...but why would one device link to multiple viommu instances?
> 
> That's a suggestion from Jason, IIRC, to avoid limiting a device
> to a single viommu, though I can't find out the source at this
> moment.
> 
> Jason, would you mind shed some light here?

We could probably do it either way, it just doesn't seem like there is
a strong reason to limit a device to a single viommu even if in
practice that is how everyone will use it.

Jason