linux-kernel - Re: [RFC PATCHES 00/17] IOMMUFD: Deliver IO page faults to user space

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Date:   Mon, 26 Jun 2023 11:02:06 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
CC:     Jason Gunthorpe <jgg@...pe.ca>, Kevin Tian <kevin.tian@...el.com>,
        "Joerg Roedel" <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Yi Liu" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        <iommu@...ts.linux.dev>, <linux-kselftest@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCHES 00/17] IOMMUFD: Deliver IO page faults to user space

On Mon, Jun 26, 2023 at 11:10:22AM +0800, Baolu Lu wrote:

> > > > I think that, whether the guest has an IOPF capability or not,
> > > > the host should always forward any stage-1 fault/error back to
> > > > the guest. Yet, the implementation of this series builds with
> > > > the IOPF framework that doesn't report IOMMU_FAULT_DMA_UNRECOV.
> > > > 
> > > > And I have my doubt at the using the IOPF framework with that
> > > > IOMMU_PAGE_RESP_ASYNC flag: using the IOPF framework is for
> > > > its bottom half workqueue, because a page response could take
> > > > a long cycle. But adding that flag feels like we don't really
> > > > need the bottom half workqueue, i.e. losing the point of using
> > > > the IOPF framework, IMHO.
> > > > 
> > > > Combining the two facts above, I wonder if we really need to
> > > > go through the IOPF framework; can't we just register a user
> > > > fault handler in the iommufd directly upon a valid event_fd?
> > > Agreed. We should avoid workqueue in sva iopf framework. Perhaps we
> > > could go ahead with below code? It will be registered to device with
> > > iommu_register_device_fault_handler() in IOMMU_DEV_FEAT_IOPF enabling
> > > path. Un-registering in the disable path of cause.
> > Well, for a virtualization use case, I still think it's should
> > be registered in iommufd.
> 
> Emm.. you suggest iommufd calls iommu_register_device_fault_handler() to
> register its own page fault handler, right?
> 
> I have a different opinion, iommu_register_device_fault_handler() is
> called to register a fault handler for a device. It should be called
> or initiated by a device driver. The iommufd only needs to install a
> per-domain io page fault handler.
> 
> I am considering a use case on Intel platform. Perhaps it's similar
> on other platforms. An SIOV-capable device can support host SVA and
> assigning mediated devices to user space at the same time. Both host
> SVA and mediated devices require IOPF. So there will be multiple places
> where a page fault handler needs to be registered.

Okay, the narrative makes sense to me. I was more thinking of
the nesting case. The iommu_register_device_fault_handler() is
registered per device, as its name implies, while the handler
probably should be slightly different by the attaching domain.

It seems that your io_pgfault_handler() in the previous email
can potentially handle this, i.e. a IOMMU_DOMAIN_NESTED could
set domain->iopf_handler to forward DMA faults to user space.
We just need to make sure this pathway would be unconditional
at the handler registration and fault->type.

> > Having a device without an IOPF/PRI
> > capability, a guest OS should receive some faults too, if that
> > device causes a translation failure.
> 
> Yes. DMA faults are also a consideration. But I would like to have it
> supported in a separated series. As I explained in the previous reply,
> we also need to consider the software nested translation case.

I see.

Thanks
Nic