linux-kernel - Re: [PATCH v4 1/9] iommu: Introduce domain attachment handle

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Date: Tue, 9 Apr 2024 09:34:12 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: baolu.lu@...ux.intel.com, Kevin Tian <kevin.tian@...el.com>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>,
 Jean-Philippe Brucker <jean-philippe@...aro.org>,
 Nicolin Chen <nicolinc@...dia.com>, Yi Liu <yi.l.liu@...el.com>,
 Jacob Pan <jacob.jun.pan@...ux.intel.com>,
 Joel Granados <j.granados@...sung.com>, iommu@...ts.linux.dev,
 virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/9] iommu: Introduce domain attachment handle

On 4/8/24 10:05 PM, Jason Gunthorpe wrote:
> On Sat, Apr 06, 2024 at 12:34:14PM +0800, Baolu Lu wrote:
>> On 4/3/24 7:58 PM, Jason Gunthorpe wrote:
>>> On Wed, Apr 03, 2024 at 09:15:11AM +0800, Lu Baolu wrote:
>>>> Currently, when attaching a domain to a device or its PASID, domain is
>>>> stored within the iommu group. It could be retrieved for use during the
>>>> window between attachment and detachment.
>>>>
>>>> With new features introduced, there's a need to store more information
>>>> than just a domain pointer. This information essentially represents the
>>>> association between a domain and a device. For example, the SVA code
>>>> already has a custom struct iommu_sva which represents a bond between
>>>> sva domain and a PASID of a device. Looking forward, the IOMMUFD needs
>>>> a place to store the iommufd_device pointer in the core, so that the
>>>> device object ID could be quickly retrieved in the critical fault handling
>>>> path.
>>>>
>>>> Introduce domain attachment handle that explicitly represents the
>>>> attachment relationship between a domain and a device or its PASID.
>>>> A caller-specific data field can be used by the caller to store additional
>>>> information beyond a domain pointer, depending on its specific use case.
>>>>
>>>> Co-developed-by: Jason Gunthorpe<jgg@...dia.com>
>>>> Signed-off-by: Jason Gunthorpe<jgg@...dia.com>
>>>> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> ---
>>>>    drivers/iommu/iommu-priv.h |   9 +++
>>>>    drivers/iommu/iommu.c      | 158 +++++++++++++++++++++++++++++++++----
>>>>    2 files changed, 153 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/iommu/iommu-priv.h b/drivers/iommu/iommu-priv.h
>>>> index 5f731d994803..08c0667cef54 100644
>>>> --- a/drivers/iommu/iommu-priv.h
>>>> +++ b/drivers/iommu/iommu-priv.h
>>>> @@ -28,4 +28,13 @@ void iommu_device_unregister_bus(struct iommu_device *iommu,
>>>>    				 const struct bus_type *bus,
>>>>    				 struct notifier_block *nb);
>>>> +struct iommu_attach_handle {
>>>> +	struct iommu_domain		*domain;
>>>> +	refcount_t			users;
>>> I don't understand how the refcounting can be generally useful. There
>>> is no way to free this:
>>>
>>>> +	void				*priv;
>>> When the refcount goes to zero.
>> This field is set by the caller, so the caller ensures that the pointer
>> can only be freed after iommu domain detachment. For iopf, the caller
>> should automatically respond to all outstanding iopf's in the domain
>> detach path.
>>
>> In the sva case, which uses the workqueue to handle iopf,
>> flush_workqueue() is called in the domain detach path to ensure that all
>> outstanding iopf's are completed before detach completion.
> Which is back to what is the point of the refcount at all?

Yeah, refcount is not generally useful. It's context-specific, so it
needs to move out of the core.

The SVA code needs refcount because it allows multiple attachments
between a SVA domain and a PASID. This is not a common case.

> 
>> +static void iommufd_auto_response_handle(struct iommufd_fault *fault,
>> +                                        struct iommu_attach_handle *handle)
>> +{
>> +       struct iommufd_device *idev = handle->priv;
> The caller already has the iommufd_device, don't need the handler.

Yes.

Best regards,
baolu