linux-kernel - Re: [PATCH v4 03/13] PCI: pci_stub: Suppress kernel DMA ownership auto-claiming

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Message-ID: <9daec0aa-b58f-93b3-c8bb-b67ec6d84596@linux.intel.com>
Date:   Fri, 31 Dec 2021 09:58:11 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     baolu.lu@...ux.intel.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        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 v4 03/13] PCI: pci_stub: Suppress kernel DMA ownership
 auto-claiming

On 12/31/21 9:10 AM, Lu Baolu wrote:
> 
> On 12/31/21 8:40 AM, Jason Gunthorpe wrote:
>> On Thu, Dec 30, 2021 at 04:24:14PM -0600, Bjorn Helgaas wrote:
>>
>>> I was speculating that maybe the DMA ownership claiming must be done
>>> *before* the driver's .probe() method?
>>
>> This is correct.
>>
>>> If DMA ownership could be claimed by the .probe() method, we
>>> wouldn't need the new flag in struct device_driver.
>>
>> The other requirement is that every existing driver must claim
>> ownership, so pushing this into the device driver's probe op would
>> require revising almost every driver in Linux...
>>
>> In effect the new flag indicates if the driver will do the DMA
>> ownership claim in it's probe, or should use the default claim the
>> core code does.
>>
>> In almost every case a driver should do a claim. A driver like
>> pci-stub, or a bridge, that doesn't actually operate MMIO on the
>> device would be the exception.
> 
> We still need to call iommu_device_use_dma_api() in bus dma_configure()
> callback. But we can call iommu_device_unuse_dma_api() in the .probe()
> of vfio (and vfio-approved) drivers, so that we don't need the new flag
> anymore.

Oh, wait. I didn't think about the hot-plug case. If we call
iommu_device_use_dma_api() in bus dma_configure() anyway, we can't bind
any (no matter vfio or none-vfio) driver to a device if it's group has
already been assigned to user space. It seems that we can't omit this
flag.

Best regards,
baolu