linux-kernel - Re: [PATCH v2 08/16] iommu: define and export iommu_access_remote

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Date:   Wed, 8 Mar 2023 10:23:48 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Fenghua Yu <fenghua.yu@...el.com>, Vinod Koul <vkoul@...nel.org>,
        Dave Jiang <dave.jiang@...el.com>
Cc:     baolu.lu@...ux.intel.com, dmaengine@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alistair Popple <apopple@...dia.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>, iommu@...ts.linux.dev
Subject: Re: [PATCH v2 08/16] iommu: define and export
 iommu_access_remote_vm()

On 3/8/23 1:55 AM, Fenghua Yu wrote:
> Hi, Baolu,
> 
> On 3/6/23 17:41, Baolu Lu wrote:
>> On 3/7/23 12:31 AM, Fenghua Yu wrote:
>>> Define and export iommu_access_remote_vm() to allow IOMMU related
>>> drivers to access user address space by PASID.
>>>
>>> The IDXD driver would like to use it to write the user's completion
>>> record that the hardware device is not able to write to due to user
>>> page fault.
>>
>> I don't quite follow here. Isn't I/O page fault already supported?
> 
> The following patch 9 in this series explains in details why IDXD device 
> cannot use page fault to write to user memory: 
> https://lore.kernel.org/dmaengine/20230306163138.587484-10-fenghua.yu@intel.com/
> 
> "DSA supports page fault handling through PRS. However, the DMA engine
> that's processing the descriptor is blocked until the PRS response is
> received. Other workqueues sharing the engine are also blocked.
> Page fault handing by the driver with PRS disabled can be used to
> mitigate the stalling.

Ah! Get your point now. Thanks for the explanation.

> 
> With PRS disabled while ATS remain enabled, DSA handles page faults on
> a completion record by reporting an event in the event log. In this
> instance, the descriptor is completed and the event log contains the
> completion record address and the contents of the completion record."
> 
> That's why IDXD driver needs this IOMMU's helper 
> iommu_access_remote_vm() to copy the completion record from event log 
> buffer to user space.
> 
> Thanks.
> 
> -Fenghua

Best regards,
baolu