linux-kernel - Re: [PATCH v5 01/14] iommu: Add dma ownership management interfaces

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Date:   Thu, 6 Jan 2022 11:46:35 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        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>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Dan Williams <dan.j.williams@...el.com>, rafael@...nel.org,
        Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Stuart Yoder <stuyoder@...il.com>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Li Yang <leoyang.li@....com>,
        Dmitry Osipenko <digetx@...il.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 01/14] iommu: Add dma ownership management interfaces

On Thu, Jan 06, 2022 at 11:54:06AM +0800, Lu Baolu wrote:
> On 1/5/22 3:23 AM, Jason Gunthorpe wrote:
> > > > > The device driver oriented interfaces are,
> > > > > 
> > > > > 	int iommu_device_use_dma_api(struct device *dev);
> > > > > 	void iommu_device_unuse_dma_api(struct device *dev);
> > > Nit, do we care whether it uses the actual DMA API?  Or is it just
> > > that iommu_device_use_dma_api() tells us the driver may program the
> > > device to do DMA?
> > As the main purpose, yes this is all about the DMA API because it
> > asserts the group domain is the DMA API's domain.
> > 
> > There is a secondary purpose that has to do with the user/kernel
> > attack you mentioned above. Maintaining the DMA API domain also
> > prevents VFIO from allowing userspace to operate any device in the
> > group which blocks P2P attacks to MMIO of other devices.
> > 
> > This is why, even if the driver doesn't use DMA, it should still do a
> > iommu_device_use_dma_api(), except in the special cases where we don't
> > care about P2P attacks (eg pci-stub, bridges, etc).
> > 
> 
> By the way, use_dma_api seems hard to read. How about
> 
> 	iommu_device_use_default_dma()?

You could just say "use default domain"

IMHO the way the iommu subsystem has its own wonky language is a
little troublesome. In the rest of the kernel we call this the DMA
API, while the iommu subsystem calls the domain that the DMA API uses
the 'default domain' not the 'DMA API' domain.

Still, it is probably better to align to the iommu language - just be
sure to put in the function comment that this API 'allows the driver
to use the DMA API eg dma_map_sg()'

Jason