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

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Message-ID: <c6a011c4-e3d7-4f6a-a3bc-2000e415db74@amd.com>
Date: Tue, 6 May 2025 14:35:07 +1000
From: Alexey Kardashevskiy <aik@....com>
To: Nicolin Chen <nicolinc@...dia.com>, Jason Gunthorpe <jgg@...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, 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 6/5/25 12:53, Nicolin Chen wrote:
> 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?

Yup, this sounds about right, recreate on the bus number change.

> 
> Thanks
> Nicolin

-- 
Alexey