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

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Message-ID: <20241116003443.GB35230@nvidia.com>
Date: Fri, 15 Nov 2024 20:34:43 -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 Fri, Nov 15, 2024 at 02:07:41PM -0800, Nicolin Chen wrote:
> On Thu, Nov 14, 2024 at 12:20:10PM -0400, Jason Gunthorpe wrote:
> > 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.
> 
> Intel's virtual IOMMU in QEMU has one instance but could create
> two vIOMMU objects for devices behind two different pIOMMUs. So,
> in this case, it does the on-demand (or try-and-fail) approach?

I suspect Intel does need viommu at all, and if it ever does it will
not be able to have one instance..

> One corner case that Yi reminded me of was that VMM having two
> virtual IOMMUs for two devices that are behind the same pIOMMU,
> then these two virtual IOMMUs don't necessarily share the same
> vIOMMU object, i.e. VMM is allowed to allocate two vIOMMU objs?

Yes this is allowed
 
> > 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.
> 
> By "auto-provision", you mean libvirt assigning devices to the
> correct virtual IOMMUs corresponding to the physical instances?
> If so, we can just match the "iommu" sysfs node of devices with
> the iommu node(s) under /sys/class/iommu/, right?

Yes

Jason