linux-kernel - Re: [PATCH v3 01/18] iommu: Add device dma ownership set/release interfaces

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Message-ID: <Ya4Ru/GtILJYzI6j@8bytes.org>
Date:   Mon, 6 Dec 2021 14:35:55 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...dia.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 v3 01/18] iommu: Add device dma ownership set/release
 interfaces

On Mon, Dec 06, 2021 at 09:58:46AM +0800, Lu Baolu wrote:
> >From the perspective of who is initiating the device to do DMA, device
> DMA could be divided into the following types:
> 
>         DMA_OWNER_DMA_API: Device DMAs are initiated by a kernel driver
> 			through the kernel DMA API.
>         DMA_OWNER_PRIVATE_DOMAIN: Device DMAs are initiated by a kernel
> 			driver with its own PRIVATE domain.
> 	DMA_OWNER_PRIVATE_DOMAIN_USER: Device DMAs are initiated by
> 			userspace.

I have looked at the other iommu patches in this series, but I still
don't quite get what the difference in the code flow is between
DMA_OWNER_PRIVATE_DOMAIN and DMA_OWNER_PRIVATE_DOMAIN_USER. What are the
differences in the iommu core behavior based on this setting?

>         int iommu_device_set_dma_owner(struct device *dev,
>                 enum iommu_dma_owner type, void *owner_cookie);
>         void iommu_device_release_dma_owner(struct device *dev,
>                 enum iommu_dma_owner type);

It the owner is a group-wide setting, it should be called with the group
instead of the device. I have seen the group-specific funcitons are
added later, but that leaves the question why the device-specific ones
are needed at all.

> +	enum iommu_dma_owner dma_owner;
> +	refcount_t owner_cnt;
> +	void *owner_cookie;
>  };

I am also not quite happy yet with calling this dma_owner, but can't
come up with a better name yet.

>  
>  struct group_device {
> @@ -621,6 +624,7 @@ struct iommu_group *iommu_group_alloc(void)
>  	INIT_LIST_HEAD(&group->devices);
>  	INIT_LIST_HEAD(&group->entry);
>  	BLOCKING_INIT_NOTIFIER_HEAD(&group->notifier);
> +	group->dma_owner = DMA_OWNER_NONE;


DMA_OWNER_NONE is also questionable. All devices are always in one
domain, and the default domain is always the one used for DMA-API, so
why isn't the initial value DMA_OWNER_DMA_API?

Regards,

	Joerg