linux-kernel - Re: [PATCH v12 13/16] iommu: Improve iopf_queue_remove

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Date: Thu, 8 Feb 2024 09:32:05 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Vasant Hegde <vasant.hegde@....com>, "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>,
 Longfang Liu <liulongfang@...wei.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
 Joel Granados <j.granados@...sung.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>,
 Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v12 13/16] iommu: Improve iopf_queue_remove_device()

On 2024/2/8 1:59, Vasant Hegde wrote:
> Hi Baolu,
> 
> On 2/7/2024 5:59 PM, Baolu Lu wrote:
>> On 2024/2/7 10:50, Tian, Kevin wrote:
>>>> From: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> Sent: Wednesday, February 7, 2024 9:33 AM
>>>>
>>>> Convert iopf_queue_remove_device() to return void instead of an error code,
>>>> as the return value is never used. This removal helper is designed to be
>>>> never-failed, so there's no need for error handling.
>>>>
>>>> Ack all outstanding page requests from the device with the response code of
>>>> IOMMU_PAGE_RESP_INVALID, indicating device should not attempt any retry.
>>>>
>>>> Add comments to this helper explaining the steps involved in removing a
>>>> device from the iopf queue and disabling its PRI. The individual drivers
>>>> are expected to be adjusted accordingly. Here we just define the expected
>>>> behaviors of the individual iommu driver from the core's perspective.
>>>>
>>>> Suggested-by: Jason Gunthorpe<jgg@...dia.com>
>>>> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> Reviewed-by: Jason Gunthorpe<jgg@...dia.com>
>>>> Tested-by: Yan Zhao<yan.y.zhao@...el.com>
>>> Reviewed-by: Kevin Tian<kevin.tian@...el.com>, with one nit:
>>>
>>>> + * Removing a device from an iopf_queue. It's recommended to follow
>>>> these
>>>> + * steps when removing a device:
>>>>     *
>>>> - * Return: 0 on success and <0 on error.
>>>> + * - Disable new PRI reception: Turn off PRI generation in the IOMMU
>>>> hardware
>>>> + *   and flush any hardware page request queues. This should be done
>>>> before
>>>> + *   calling into this helper.
>>>> + * - Acknowledge all outstanding PRQs to the device: Respond to all
>>>> outstanding
>>>> + *   page requests with IOMMU_PAGE_RESP_INVALID, indicating the device
>>>> should
>>>> + *   not retry. This helper function handles this.
>>> this implies calling iopf_queue_remove_device() here.
>>>
>>>> + * - Disable PRI on the device: After calling this helper, the caller could
>>>> + *   then disable PRI on the device.
>>>> + * - Call iopf_queue_remove_device(): Calling iopf_queue_remove_device()
>>>> + *   essentially disassociates the device. The fault_param might still exist,
>>>> + *   but iommu_page_response() will do nothing. The device fault parameter
>>>> + *   reference count has been properly passed from
>>>> iommu_report_device_fault()
>>>> + *   to the fault handling work, and will eventually be released after
>>>> + *   iommu_page_response().
>>>>     */
>>> but here it suggests calling iopf_queue_remove_device() again. If the comment
>>> is just about to detail the behavior with that invocation shouldn't it be merged
>>> with the previous one instead of pretending to be the final step for driver
>>> to call?
>>
>> Above just explains the behavior of calling iopf_queue_remove_device().
> 
> Can you please leave a line -OR- move this to previous para? Otherwise we will
> get confused.

Sure. I will make it look like below.

/**
  * iopf_queue_remove_device - Remove producer from fault queue
  * @queue: IOPF queue
  * @dev: device to remove
  *
  * Removing a device from an iopf_queue. It's recommended to follow these
  * steps when removing a device:
  *
  * - Disable new PRI reception: Turn off PRI generation in the IOMMU 
hardware
  *   and flush any hardware page request queues. This should be done before
  *   calling into this helper.
  * - Acknowledge all outstanding PRQs to the device: Respond to all 
outstanding
  *   page requests with IOMMU_PAGE_RESP_INVALID, indicating the device 
should
  *   not retry. This helper function handles this.
  * - Disable PRI on the device: After calling this helper, the caller could
  *   then disable PRI on the device.
  *
  * Calling iopf_queue_remove_device() essentially disassociates the device.
  * The fault_param might still exist, but iommu_page_response() will do
  * nothing. The device fault parameter reference count has been properly
  * passed from iommu_report_device_fault() to the fault handling work, and
  * will eventually be released after iommu_page_response().
  */

Best regards,
baolu