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

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Date:   Wed, 17 Nov 2021 09:35:17 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>, kvm@...r.kernel.org,
        rafael@...nel.org, linux-pci@...r.kernel.org,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 01/11] iommu: Add device dma ownership set/release
 interfaces

On Wed, Nov 17, 2021 at 01:22:19PM +0800, Lu Baolu wrote:
> Hi Jason,
> 
> On 11/16/21 9:46 PM, Jason Gunthorpe wrote:
> > On Tue, Nov 16, 2021 at 09:57:30AM +0800, Lu Baolu wrote:
> > > Hi Christoph,
> > > 
> > > On 11/15/21 9:14 PM, Christoph Hellwig wrote:
> > > > On Mon, Nov 15, 2021 at 10:05:42AM +0800, Lu Baolu wrote:
> > > > > +enum iommu_dma_owner {
> > > > > +	DMA_OWNER_NONE,
> > > > > +	DMA_OWNER_KERNEL,
> > > > > +	DMA_OWNER_USER,
> > > > > +};
> > > > > +
> > > > 
> > > > > +	enum iommu_dma_owner dma_owner;
> > > > > +	refcount_t owner_cnt;
> > > > > +	struct file *owner_user_file;
> > > > 
> > > > I'd just overload the ownership into owner_user_file,
> > > > 
> > > >    NULL			-> no owner
> > > >    (struct file *)1UL)	-> kernel
> > > >    real pointer		-> user
> > > > 
> > > > Which could simplify a lot of the code dealing with the owner.
> > > > 
> > > 
> > > Yeah! Sounds reasonable. I will make this in the next version.
> > 
> > It would be good to figure out how to make iommu_attach_device()
> > enforce no other driver binding as a kernel user without a file *, as
> > Robin pointed to, before optimizing this.
> > 
> > This fixes an existing bug where iommu_attach_device() only checks the
> > group size and is vunerable to a hot plug increasing the group size
> > after it returns. That check should be replaced by this series's logic
> > instead.
> 
> As my my understanding, the essence of this problem is that only the
> user owner of the iommu_group could attach an UNMANAGED domain to it.
> If I understand it right, how about introducing a new interface to
> allocate a user managed domain and storing the user file pointer in it.

For iommu_attach_device() the semantic is simple non-sharing, so there
is no need for the file * at all, it can just be NULL.

> Does above help here?

No, iommu_attach_device() is kernel only and should not interact with
userspace.

I'm also going to see if I can learn what Tegra is doing with
iommu_attach_group()

Jason