linux-kernel - Re: [PATCH v4 09/10] iommu: Make iommu_queue

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Message-ID: <b48e14ed-693a-8c88-3391-76cacb0850b9@linux.intel.com>
Date:   Wed, 13 Sep 2023 12:23:31 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>
Cc:     baolu.lu@...ux.intel.com, "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 09/10] iommu: Make iommu_queue_iopf() more generic

On 9/13/23 10:34 AM, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Monday, September 11, 2023 8:46 PM
>>
>> On 2023/9/11 14:57, Tian, Kevin wrote:
>>>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>>>> Sent: Tuesday, September 5, 2023 1:24 PM
>>>>
>>>> Hi Kevin,
>>>>
>>>> I am trying to address this issue in below patch. Does it looks sane to
>>>> you?
>>>>
>>>> iommu: Consolidate per-device fault data management
>>>>
>>>> The per-device fault data is a data structure that is used to store
>>>> information about faults that occur on a device. This data is allocated
>>>> when IOPF is enabled on the device and freed when IOPF is disabled. The
>>>> data is used in the paths of iopf reporting, handling, responding, and
>>>> draining.
>>>>
>>>> The fault data is protected by two locks:
>>>>
>>>> - dev->iommu->lock: This lock is used to protect the allocation and
>>>>      freeing of the fault data.
>>>> - dev->iommu->fault_parameter->lock: This lock is used to protect the
>>>>      fault data itself.
>>>>
>>>> Improve the iopf code to enforce this lock mechanism and add a
>> reference
>>>> counter in the fault data to avoid use-after-free issue.
>>>>
>>>
>>> Can you elaborate the use-after-free issue and why a new user count
>>> is required?
>>
>> I was concerned that when iommufd uses iopf, page fault report/response
>> may occur simultaneously with enable/disable PRI.
>>
>> Currently, this is not an issue as the enable/disable PRI is in its own
>> path. In the future, we may discard this interface and enable PRI when
>> attaching the first PRI-capable domain, and disable it when detaching
>> the last PRI-capable domain.
> 
> Then let's not do it now until there is a real need after you have a
> thorough design for iommufd.

Okay, fair enough.

> 
>>
>>>
>>> btw a Fix tag is required given this mislocking issue has been there for
>>> quite some time...
>>
>> I don't see any real issue fixed by this change. It's only a lock
>> refactoring after the code refactoring and preparing it for iommufd use.
>> Perhaps I missed anything?
>>
> 
> mislocking already exists today for the partial list:
> 
>    - iommu_queue_iopf() uses dev->iommu->lock;
>    - iopf_queue_discard_partial() uses queue->lock;

So, if it's worth it, let me try splitting a fix patch.

Best regards,
baolu