linux-kernel - Re: [PATCH v7 13/13] Documentation: userspace-api: iommufd: Update vIOMMU

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Message-ID: <20241114162010.GP35230@nvidia.com>
Date: Thu, 14 Nov 2024 12:20:10 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: Yi Liu <yi.l.liu@...el.com>, kevin.tian@...el.com, corbet@....net,
	joro@...tes.org, suravee.suthikulpanit@....com, will@...nel.org,
	robin.murphy@....com, dwmw2@...radead.org, shuah@...nel.org,
	iommu@...ts.linux.dev, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	baolu.lu@...ux.intel.com, eric.auger@...hat.com,
	jean-philippe@...aro.org, mdf@...nel.org, mshavit@...gle.com,
	shameerali.kolothum.thodi@...wei.com, smostafa@...gle.com,
	aik@....com, zhangfei.gao@...aro.org, patches@...ts.linux.dev
Subject: Re: [PATCH v7 13/13] Documentation: userspace-api: iommufd: Update
 vIOMMU

On Wed, Nov 13, 2024 at 07:18:42PM -0800, Nicolin Chen wrote:
> > so the user would try to create vDevices with a given viommu_obj until
> > failure, then it would allocate another viommu_obj for the failed device.
> > is it? sounds reasonable.
> 
> Yes. It is the same as previously dealing with a nesting parent:
> test and allocate if fails. The virtual IOMMU driver in VMM can
> keep a list of the vIOMMU objects for each device to test.

The viommu object should be tied to the VMM's vIOMMU vHW object that
it is paravirtualizing toward the VM.

So we shouldn't be creating viommu objects on demand, it should be
created when the vIOMMU is created, and the presumably the qemu
command line will describe how to link vPCI/VFIO functions to vIOMMU
instances. If they kernel won't allow the user's configuration then it
should fail, IMHO.

Some try-and-fail might be interesting to auto-provision vIOMMU's and
provision vPCI functions. Though I suspect we will be providing
information in other ioctls so something like libvirt can construct
the correct configuration directly.

Jason