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

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Message-ID: <Zva_nFXZgT4MEfDg@google.com>
Date: Fri, 27 Sep 2024 14:22:20 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Nicolin Chen <nicolinc@...dia.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, yi.l.liu@...el.com
Subject: Re: [PATCH v2 06/19] iommufd/viommu: Add
 IOMMU_VIOMMU_SET/UNSET_VDEV_ID ioctl

On Fri, Sep 27, 2024 at 11:01:41AM -0300, Jason Gunthorpe wrote:
> On Fri, Sep 27, 2024 at 01:50:52PM +0000, Mostafa Saleh wrote:
> 
> > My understanding of IOMMUFD is very little, but AFAICT, that means that
> > it’s assumed that each device can only have one stream ID(RID)?
> > 
> > As I can see in patch 17 in arm_smmu_convert_viommu_vdev_id(), it
> > converts the virtual ID to a physical one using master->streams[0].id.
> > 
> > Is that correct or am I missing something?
> > 
> > As I am looking at similar problem for paravirtual IOMMU with pKVM, where
> > the UAPI would be something similar to:
> > 
> > 	GET_NUM_END_POINTS(dev) => nr_sids
> > 
> > 	SET_END_POINT_VSID(dev, sid_index, vsid)
> > 
> > Similar to what VFIO does with IRQs.
> > 
> > As a device can have many SIDs.
> 
> We don't support multi SID through this interface, at least in this
> version.
> 
> To do multi-sid you have to inform the VM of all the different pSIDs
> the device has and then setup the vSID/pSID translation to map them
> all to the HW invalidation logic.

Why would the VM need to know the pSID? The way I view this is quite close to
how irq works, the VM only views the GSI which is the virtualized number.
The VMM then would need to configure vSID->pSID translation, also without
knowing the actual pSID, just how many SIDs are there per-device; very similar to
how it configures IRQs through
VFIO_DEVICE_GET_INFO/VFIO_DEVICE_SET_IRQS.

And as long as we only allow 1:1 vSID to pSID mapping, I guess it would be
easy to implement.

> 
> Which is alot more steps, and we have no use case right now. Multi-sid
> is also not something I expect to see in any modern PCI device, and
> this is VFIO PCI...
> 

Ah, I thought IOMMUFD would be used instead of VFIO_TYPE1*, which should cover
platform devices (VFIO-platform) or am I missing something?

And multi-SIDs is common in platform devices and this would be quite
restricting, and I was hoping to support the pKVM vIOMMU through IOMMUFD
interface.

If possible, can the UAPI be designed with this in mind, even if not
implemented now?

Thanks,
Mostafa

> Jason