linux-kernel - Re: [PATCH v2 06/19] iommufd/viommu: Add IOMMU_VIOMMU_SET/UNSET_VDEV

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Message-ID: <aBl5uLOFCntuIYYz@nvidia.com>
Date: Mon, 5 May 2025 19:53:44 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Alexey Kardashevskiy <aik@....com>, <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>,
	<eric.auger@...hat.com>, <jean-philippe@...aro.org>, <mdf@...nel.org>,
	<mshavit@...gle.com>, <shameerali.kolothum.thodi@...wei.com>,
	<smostafa@...gle.com>, <yi.l.liu@...el.com>
Subject: Re: [PATCH v2 06/19] iommufd/viommu: Add
 IOMMU_VIOMMU_SET/UNSET_VDEV_ID ioctl

On Mon, May 05, 2025 at 02:08:07PM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 30, 2025 at 12:58:47AM -0700, Nicolin Chen wrote:
> 
> > > ... and I just hit a problem with it - this is basically guest BDFn
> > > and it works as long as I'm hotplugging the TEE-IO VF into an SNP VM
> > > but does not when I pass through via the QEMU cmdline - bus numbers
> > > are not assigned yet. So I have to postpone the vdevice allocation
> > > till run time, did I miss something here? Thanks,
> > 
> > I have a similar case with QEMU ARM64's VM: so vDEVICE on ARM is
> > allocated at runtime as well because the BDF number isn't ready
> > at the boot time.
> 
> Oh that's ugly then.. So you'll need to add some kind of 'modify
> sid/bdf' operation I think.

But the initial vDEVICE would be still unusable. Its BDF number is
literally 0 in my case. It can't be used for SID-based invalidation
nor the reverse vSID lookup for fault injection..

> The bus numbers can be reassigned at any time on the fly by the guest
> by reprogramming the PCI hierarchy.

Yes. If we take some aggressive use case into account, where its
BDF number could change multiple times, I think it's natural for
VMM to simply destroy the previous vDEVICE and allocate a new one
with a new BDF number, right?

Thanks
Nicolin