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

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Message-ID: <ZwBBH+9hwuGKOstl@Asurada-Nvidia>
Date: Fri, 4 Oct 2024 12:25:19 -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 Fri, Oct 04, 2024 at 03:50:19PM -0300, Jason Gunthorpe wrote:
> On Fri, Oct 04, 2024 at 11:13:46AM -0700, Nicolin Chen wrote:
> > On Fri, Oct 04, 2024 at 08:41:47AM -0300, Jason Gunthorpe wrote:
> > > On Fri, Oct 04, 2024 at 02:32:28PM +1000, Alexey Kardashevskiy wrote:
> > > > For my SEV-TIO exercise ("trusted IO"), I am looking for a kernel interface
> > > > to pass the guest's BDFs for a specific host device (which is passed
> > > > through) and nothing in the kernel has any knowledge of it atm, is this the
> > > > right place, or another ioctl() is needed here?
> > > 
> > > We probably need to add the vRID as well to this struct for that
> > > reason.
> > 
> > "vRID"/"vBDF" doesn't sound very generic to me to put in this
> > structure, though PCI devices are and very likely will be the
> > only users of this Virtual Device for a while. Any good idea?
> 
> It isn't necessarily bad to have a pci field as long as we can
> somehow understand when it is used.

OK.

> > Also, I am wondering if the uAPI structure of Virtual Device
> > should have a driver-specific data structure. And the vdev_id
> > should be in the driver-specific struct. So, it could stay in
> > corresponding naming, "Stream ID", "Device ID" or "Context ID"
> > v.s. a generic "Virtual ID" in the top-level structure? Then,
> > other info like CCA can be put in the driver-level structure
> > of SMMU's.
> 
> I'd to avoid a iommu-driver specific structure here, but I fear we
> will have a "lowervisor" (sigh) specific structure for the widely
> varied CC/pkvm/etc world.

The design of the structure also impacts how we implement the
API between iommufd and the drivers. Right now, forwarding the
ID via a function parameter is fine, but we would need a user
structure once we have more stuff to forward.

With that, I wonder what is better for the initial version of
this structure, a generic virtual ID or a driver-named ID like
"Stream ID"? The latter might be more understandable/flexible, 
so we won't need to justify a generic virtual ID along the way
if something changes in the nature?

> > Agreed. That also implies that a vRID is quite independent to
> > the IOMMU right? So, I think that the reason of adding a vRID
> > to the virtual deivce uAPI/structure should be IOMMU requiring
> > it?
> 
> I would like to use this API to link in the CC/pkvm/etc world, and use
> it to create not just the vIOMMU components but link up to the
> "lowervisor" components as well, since it is all the same stuff
> basically.

That sounds wider than what I defined it for in my patch:
 * struct iommu_vdevice_alloc - ioctl(IOMMU_VDEVICE_ALLOC)
 * ...
 * Allocate a virtual device instance (for a physical device) against a vIOMMU.
 * This instance holds the device's information in a VM, related to its vIOMMU.

Would you please help rephrase it? It'd be also helpful for me
to update the doc.

Though I feel slightly odd if we define it wider than "vIOMMU"
since this is an iommufd header...

Thanks
Nicolin