Re: [PATCH v2 08/16] iommu: define and export iommu_access_remote_vm()

[Baolu Lu]

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@xxxxxxxxx/
>
> "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