linux-kernel - Re: [PATCH v5 5/9] iommufd: Add iommufd fault object

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Date: Mon, 20 May 2024 11:28:30 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: "Tian, Kevin" <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: baolu.lu@...ux.intel.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>, "Liu, Yi L" <yi.l.liu@...el.com>,
 Jacob Pan <jacob.jun.pan@...ux.intel.com>,
 Joel Granados <j.granados@...sung.com>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "virtualization@...ts.linux-foundation.org"
 <virtualization@...ts.linux-foundation.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 5/9] iommufd: Add iommufd fault object

On 5/20/24 11:26 AM, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Monday, May 20, 2024 8:41 AM
>>
>> On 5/15/24 3:57 PM, Tian, Kevin wrote:
>>>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>>>> Sent: Wednesday, May 8, 2024 6:05 PM
>>>>
>>>> On 2024/5/8 8:11, Jason Gunthorpe wrote:
>>>>> On Tue, Apr 30, 2024 at 10:57:06PM +0800, Lu Baolu wrote:
>>>>>> diff --git a/drivers/iommu/iommu-priv.h b/drivers/iommu/iommu-
>> priv.h
>>>>>> index ae65e0b85d69..1a0450a83bd0 100644
>>>>>> --- a/drivers/iommu/iommu-priv.h
>>>>>> +++ b/drivers/iommu/iommu-priv.h
>>>>>> @@ -36,6 +36,10 @@ struct iommu_attach_handle {
>>>>>>     			struct device	*dev;
>>>>>>     			refcount_t	users;
>>>>>>     		};
>>>>>> +		/* attach data for IOMMUFD */
>>>>>> +		struct {
>>>>>> +			void		*idev;
>>>>>> +		};
>>>>> We can use a proper type here, just forward declare it.
>>>>>
>>>>> But this sequence in the other patch:
>>>>>
>>>>> +       ret = iommu_attach_group(hwpt->domain, idev->igroup->group);
>>>>> +       if (ret) {
>>>>> +               iommufd_fault_iopf_disable(idev);
>>>>> +               return ret;
>>>>> +       }
>>>>> +
>>>>> +       handle = iommu_attach_handle_get(idev->igroup->group,
>>>> IOMMU_NO_PASID, 0);
>>>>> +       handle->idev = idev;
>>>>>
>>>>> Is why I was imagining the caller would allocate, because now we have
>>>>> the issue that a fault capable domain was installed into the IOMMU
>>>>> before it's handle could be fully setup, so we have a race where a
>>>>> fault could come in right between those things. Then what happens?
>>>>> I suppose we can retry the fault and by the time it comes back the
>>>>> race should resolve. A bit ugly I suppose.
>>>>
>>>> You are right. It makes more sense if the attached data is allocated and
>>>> managed by the caller. I will go in this direction and update my series.
>>>> I will also consider other review comments you have given in other
>>>> places.
>>>>
>>>
>>> Does this direction imply a new iommu_attach_group_handle() helper
>>> to pass in the caller-allocated handle pointer or exposing a new
>>> iommu_group_set_handle() to set the handle to the group pasid_array
>>> and then having iomm_attach_group() to update the domain info in
>>> the handle?
>>
>> I will add new iommu_attach/replace/detach_group_handle() helpers. Like
>> below:
>>
>> +/**
>> + * iommu_attach_group_handle - Attach an IOMMU domain to an IOMMU
>> group
>> + * @domain: IOMMU domain to attach
>> + * @group: IOMMU group that will be attached
>> + * @handle: attach handle
>> + *
>> + * Returns 0 on success and error code on failure.
>> + *
>> + * This is a variant of iommu_attach_group(). It allows the caller to
>> provide
>> + * an attach handle and use it when the domain is attached. This is
>> currently
>> + * only designed for IOMMUFD to deliver the I/O page faults.
>> + */
>> +int iommu_attach_group_handle(struct iommu_domain *domain,
>> +                             struct iommu_group *group,
>> +                             struct iommu_attach_handle *handle)
>>
> 
> "currently only designed for IOMMUFD" doesn't sound correct.
> 
> design-wise this can be used by anyone which relies on the handle.
> There is nothing tied to IOMMUFD.
> 
> s/designed for/used by/ is more accurate.

Done.

Best regards,
baolu