linux-kernel - RE: [RFC] Simple device assignment with VFIO platform

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Message-ID: <BN9PR11MB52768B9199FAEAF8B9CC378E8C762@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Mon, 30 Sep 2024 08:19:45 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Mostafa Saleh <smostafa@...gle.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
CC: Eric Auger <eric.auger@...hat.com>, Alex Williamson
	<alex.williamson@...hat.com>, "kwankhede@...dia.com" <kwankhede@...dia.com>,
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>, Quentin Perret
	<qperret@...gle.com>
Subject: RE: [RFC] Simple device assignment with VFIO platform

> From: Mostafa Saleh <smostafa@...gle.com>
> Sent: Saturday, September 28, 2024 12:17 AM
> 
> Hi All,
> 
> Background
> ==========
> I have been looking into assigning simple devices which are not DMA
> capable to VMs on Android using VFIO platform.
> 
> I have been mainly looking with respect to Protected KVM (pKVM), which
> would need some extra modifications mostly to KVM-VFIO, that is quite
> early under prototyping at the moment, which have core pending pKVM
> dependencies upstream as guest memfd[1] and IOMMUs support[2].
> 
> However, this problem is not pKVM(or KVM) specific, and about the
> design of VFIO.
> 
> [1] https://lore.kernel.org/kvm/20240801090117.3841080-1-
> tabba@...gle.com/
> [2] https://lore.kernel.org/kvmarm/20230201125328.2186498-1-jean-
> philippe@...aro.org/
> 
> Problem
> =======
> At the moment, VFIO platform will deny a device from probing (through
> vfio_group_find_or_alloc()), if it’s not part of an IOMMU group,
> unless (CONFIG_VFIO_NOIOMMU is configured)
> 
> As far as I understand the current solutions to pass through platform
> devices that are not DMA capable are:
> - Use VFIO platform + (CONFIG_VFIO_NOIOMMU): The problem with that, it
> taints the kernel and this doesn’t actually fit the device description
> as the device doesn’t only have an IOMMU, but it’s not DMA capable at
> all, so the kernel should be safe with assigning the device without
> DMA isolation.
> 
> - Use VFIO mdev with an emulated IOMMU, this seems it could work. But
> many of the code would be duplicate with the VFIO platform code as the
> device is a platform device.

emulated IOMMU is not tied to mdev:

        /*
         * Virtual device without IOMMU backing. The VFIO core fakes up an
         * iommu_group as the iommu_group sysfs interface is part of the
         * userspace ABI.  The user of these devices must not be able to
         * directly trigger unmediated DMA.
         */
        VFIO_EMULATED_IOMMU,

Except it's not a virtual device, it does match the last sentence that
such device cannot trigger unmediated DMA.

> 
> - Use UIO: Can map MMIO to userspace which seems to be focused for
> userspace drivers rather than VM passthrough and I can’t find its
> support in Qemu.
> 
> One other benefit from supporting this in VFIO platform, that we can
> use the existing UAPI for platform devices (and support in VMMs)
> 
> Proposal
> ========
> Extend VFIO platform to allow assigning devices without an IOMMU, this
> can be possibly done by
> - Checking device capability from the platform bus (would be something
> ACPI/OF specific similar to how it configures DMA from
> platform_dma_configure(), we can add a new function something like
> platfrom_dma_capable())
> 
> - Using emulated IOMMU for such devices
> (vfio_register_emulated_iommu_dev()), instead of having intrusive
> changes about IOMMUs existence.
> 
> If that makes sense I can work on RFC(I don’t have any code at the moment)

This sounds the best option out of my head now...