linux-kernel - Re: [PATCH v2 1/4] iommu: Add private_data_owner to iommu

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Message-ID: <cbf58625-e9f3-4822-9bcb-ccaadeea47bf@linux.intel.com>
Date: Fri, 28 Feb 2025 11:29:22 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, kevin.tian@...el.com, robin.murphy@....com,
 joro@...tes.org, will@...nel.org, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] iommu: Add private_data_owner to iommu_domain

On 2/28/25 11:23, Nicolin Chen wrote:
> On Fri, Feb 28, 2025 at 11:13:23AM +0800, Baolu Lu wrote:
>> On 2/28/25 09:31, Nicolin Chen wrote:
>>> Steal two bits from the 32-bit "type" in struct iommu_domain to store a
>>> new tag for private data owned by either dma-iommu or iommufd.
>>>
>>> Set the domain->private_data_owner in dma-iommu and iommufd. These will
>>> be used to replace the sw_msi function pointer.
>>>
>>> Suggested-by: Jason Gunthorpe<jgg@...dia.com>
>>> Signed-off-by: Nicolin Chen<nicolinc@...dia.com>
>>> ---
>>>    include/linux/iommu.h                | 7 ++++++-
>>>    drivers/iommu/dma-iommu.c            | 2 ++
>>>    drivers/iommu/iommufd/hw_pagetable.c | 3 +++
>>>    3 files changed, 11 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>>> index e93d2e918599..4f2774c08262 100644
>>> --- a/include/linux/iommu.h
>>> +++ b/include/linux/iommu.h
>>> @@ -209,8 +209,13 @@ struct iommu_domain_geometry {
>>>    #define IOMMU_DOMAIN_PLATFORM	(__IOMMU_DOMAIN_PLATFORM)
>>>    #define IOMMU_DOMAIN_NESTED	(__IOMMU_DOMAIN_NESTED)
>>> +#define IOMMU_DOMAIN_DATA_OWNER_NONE (0U)
>>> +#define IOMMU_DOMAIN_DATA_OWNER_DMA (1U)
>>> +#define IOMMU_DOMAIN_DATA_OWNER_IOMMUFD (2U)
>>> +
>>>    struct iommu_domain {
>>> -	unsigned type;
>>> +	u32 type : 30;
>>> +	u32 private_data_owner : 2;
>> Is there any special consideration for reserving only 2 bits for the
>> private data owner? Is it possible to allocate more bits so that it
>> could be more extensible for the future?
> It could. This "2" is copied from Jason's suggestion:
> https://lore.kernel.org/linux-iommu/20250227194749.GJ39591@nvidia.com/
> 
>> For example, current iommu core allows a kernel device driver to
>> allocate a paging domain and replace the default domain for kernel DMA.
>> This suggests the private data owner bits may be needed beyond iommu-dma
>> and iommufd.
> What's the potential "private data" in this case?

I have no idea, just feeling that we could leave room for tomorrow.