linux-kernel - Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user

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Message-ID: <9a314095-3db9-30fc-2ed9-4e46d385036d@linux.intel.com>
Date:   Tue, 28 Sep 2021 21:35:05 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Cc:     baolu.lu@...ux.intel.com, "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces

Hi Jason,

On 2021/9/28 19:57, Jason Gunthorpe wrote:
> On Tue, Sep 28, 2021 at 07:30:41AM +0000, Tian, Kevin wrote:
> 
>>> Also, don't call it "hint", there is nothing hinty about this, it has
>>> definitive functional impacts.
>>
>> possibly dma_mode (too broad?) or dma_usage
> 
> You just need a flag to specify if the driver manages DMA ownership
> itself, or if it requires the driver core to setup kernel ownership
> 
> DMA_OWNER_KERNEL
> DMA_OWNER_DRIVER_CONTROLLED
> 
> ?
> 
> There is a bool 'suprress_bind_attrs' already so it could be done like
> this:
> 
>   bool suppress_bind_attrs:1;
> 
>   /* If set the driver must call iommu_XX as the first action in probe() */
>   bool suppress_dma_owner:1;
> 
> Which is pretty low cost.

Yes. Pretty low cost to fix the BUG_ON() issue. Any kernel-DMA driver
binding is blocked if the device's iommu group has been put into user-
dma mode.

Another issue is, when putting a device into user-dma mode, all devices
belonging to the same iommu group shouldn't be bound with a kernel-dma
driver. Kevin's prototype checks this by READ_ONCE(dev->driver). This is
not lock safe as discussed below,

https://lore.kernel.org/linux-iommu/20210927130935.GZ964074@nvidia.com/

Any guidance on this?

Best regards,
baolu