linux-kernel - Re: [PATCH v1 05/16] iommufd/viommu: Add IOMMU_VIOMMU_SET/UNSET_VDEV

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Message-ID: <20240819182659.GQ2032816@nvidia.com>
Date: Mon, 19 Aug 2024 15:26:59 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v1 05/16] iommufd/viommu: Add
 IOMMU_VIOMMU_SET/UNSET_VDEV_ID ioctl

On Mon, Aug 19, 2024 at 11:10:03AM -0700, Nicolin Chen wrote:
> On Mon, Aug 19, 2024 at 02:33:32PM -0300, Jason Gunthorpe wrote:
> > On Thu, Aug 15, 2024 at 05:21:57PM -0700, Nicolin Chen wrote:
> > 
> > > > Why not? The idev becomes linked to the viommu when the dev id is set
> > > 
> > > > Unless we are also going to enforce the idev is always attached to a
> > > > nested then I don't think we need to check it here.
> > > > 
> > > > Things will definately not entirely work as expected if the vdev is
> > > > directly attached to the s2 or a blocking, but it won't harm anything.
> > > 
> > > My view is that, the moment there is a VIOMMU object, that must
> > > be a nested IOMMU case, so there must be a nested hwpt. Blocking
> > > domain would be a hwpt_nested too (vSTE=Abort) as we previously
> > > concluded.
> > 
> > I'm not sure other vendors can do that vSTE=Abort/Bypass thing though
> > yet..
> > 
> > > Then, in a nested case, it feels odd that an idev is attached to
> > > an S2 hwpt..
> > >
> > > That being said, I think we can still do that with validations:
> > >  If idev->hwpt is nested, compare input viommu v.s idev->hwpt->viommu.
> > >  If idev->hwpt is paging, compare input viommu->hwpt v.s idev->hwpt.
> > 
> > But again, if you don't contiguously validate those invariants in all
> > the other attach paths it is sort of pointless to check them since the
> > userspace can still violate things.
> 
> Hmm, would that be unsafe? I start to wonder if we should allow an
> attach to viommu and put validations on that?

I don't think it is unsafe to mismatch things, if a device is
disconnected from it's VIOMMU then the HW should isolate it the same
as anything else

It doesn't matter if the VIOMMU has a devid mapping for the device
when it is not currently part of the viommu configuration.

IOW it is not the devid ioctl that causes the device to join the
VIOMMU, it is the attach of the nest.

Jason