linux-kernel - Re: [PATCH 01/11] iommu: Add device dma ownership set/release interfaces

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Message-ID: <20211119150612.jhsvsbzisvux2lga@8bytes.org>
Date:   Fri, 19 Nov 2021 16:06:12 +0100
From:   Jörg Rödel <joro@...tes.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Diana Craciun <diana.craciun@....nxp.com>
Subject: Re: [PATCH 01/11] iommu: Add device dma ownership set/release
 interfaces

On Fri, Nov 19, 2021 at 07:14:10PM +0800, Lu Baolu wrote:
> The singleton group requirement for iommu_attach/detach_device() was
> added by below commit:
> 
> commit 426a273834eae65abcfc7132a21a85b3151e0bce
> Author: Joerg Roedel <jroedel@...e.de>
> Date:   Thu May 28 18:41:30 2015 +0200
> 
>     iommu: Limit iommu_attach/detach_device to devices with their own group
> 
>     This patch changes the behavior of the iommu_attach_device
>     and iommu_detach_device functions. With this change these
>     functions only work on devices that have their own group.
>     For all other devices the iommu_group_attach/detach
>     functions must be used.
> 
>     Signed-off-by: Joerg Roedel <jroedel@...e.de>
> 
> Joerg,can you please shed some light on the background of this
> requirement? Does above idea of transition from singleton group
> to group with single driver bound make sense to you?

This change came to be because the iommu_attach/detach_device()
interface doesn't fit well into a world with iommu-groups. Devices
within a group are by definition not isolated between each other, so
they must all be in the same address space (== iommu_domain). So it
doesn't make sense to allow attaching a single device within a group to
a different iommu_domain.

I know that in theory it is safe to allow devices within a group to be
in different domains because there iommu-groups catch multiple
non-isolation cases:

	1) Devices behind a non-ACS capable bridge or multiple functions
	   of a PCI device. Here it is safe to put the devices into
	   different iommu-domains as long as all affected devices are
	   controlled by the same owner.

	2) Devices which share a single request-id and can't be
	   differentiated by the IOMMU hardware. These always need to be
	   in the same iommu_domain.

To lift the single-domain-per-group requirement the iommu core code
needs to learn the difference between the two cases above.

Regards,

	Joerg