linux-kernel - Re: [PATCH 06/11] iommu: Expose group variants of dma ownership interfaces

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Date:   Tue, 16 Nov 2021 17:42:04 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Christoph Hellwig <hch@...radead.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>,
        Jason Gunthorpe <jgg@...dia.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 06/11] iommu: Expose group variants of dma ownership
 interfaces

Hi Christoph,

On 2021/11/15 21:27, Christoph Hellwig wrote:
> On Mon, Nov 15, 2021 at 10:05:47AM +0800, Lu Baolu wrote:
>> The vfio needs to set DMA_OWNER_USER for the entire group when attaching
> 
> The vfio subsystem?  driver?

"vfio subsystem"

> 
>> it to a vfio container. So expose group variants of setting/releasing dma
>> ownership for this purpose.
>>
>> This also exposes the helper iommu_group_dma_owner_unclaimed() for vfio
>> report to userspace if the group is viable to user assignment, for
> 
> .. for vfio to report .. ?

Yes.

> 
>>   void iommu_device_release_dma_owner(struct device *dev, enum iommu_dma_owner owner);
>> +int iommu_group_set_dma_owner(struct iommu_group *group, enum iommu_dma_owner owner,
>> +			      struct file *user_file);
>> +void iommu_group_release_dma_owner(struct iommu_group *group, enum iommu_dma_owner owner);
> 
> Pleae avoid all these overly long lines.

Sure. Thanks!

> 
>> +static inline int iommu_group_set_dma_owner(struct iommu_group *group,
>> +					    enum iommu_dma_owner owner,
>> +					    struct file *user_file)
>> +{
>> +	return -EINVAL;
>> +}
>> +
>> +static inline void iommu_group_release_dma_owner(struct iommu_group *group,
>> +						 enum iommu_dma_owner owner)
>> +{
>> +}
>> +
>> +static inline bool iommu_group_dma_owner_unclaimed(struct iommu_group *group)
>> +{
>> +	return false;
>> +}
> 
> Why do we need these stubs?  All potential callers should already
> require CONFIG_IOMMU_API?  Same for the helpers added in patch 1, btw.

You are right. This helper is only for vfio which requires IOMMU_API. I
will remove this.

The helpers in patch 1 seem not the same. The driver core (or bus
dma_configure() callback as suggested) will also call them.

> 
>> +	mutex_lock(&group->mutex);
>> +	ret = __iommu_group_set_dma_owner(group, owner, user_file);
>> +	mutex_unlock(&group->mutex);
> 
>> +	mutex_lock(&group->mutex);
>> +	__iommu_group_release_dma_owner(group, owner);
>> +	mutex_unlock(&group->mutex);
> 
> Unless I'm missing something (just skipping over the patches),
> the existing callers also take the lock just around these calls,
> so we don't really need the __-prefixed lowlevel helpers.
> 

Move mutex_lock/unlock will make the helper implementation easier. :-)
It seems to be common code style in iommu core. For example,
__iommu_attach_group(), __iommu_group_for_each_dev(), etc.

>> +	mutex_lock(&group->mutex);
>> +	owner = group->dma_owner;
>> +	mutex_unlock(&group->mutex);
> 
> No need for a lock to read a single scalar.

Adding the lock will make kcasn happy. Jason G also told me that

[citing from his review comment]
"
It is always incorrect to read concurrent data without an annotation
of some kind.

For instance it can cause mis-execution of logic where the compiler is
unaware that a value it loads is allowed to change - ie no 
READ_ONCE/WRITE_ONCE semantic.
"

> 
>> +
>> +	return owner == DMA_OWNER_NONE;
>> +}
>> +EXPORT_SYMBOL_GPL(iommu_group_dma_owner_unclaimed);

Best regards,
baolu