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

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Date:   Wed, 28 Jun 2023 09:49:31 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>,
        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 Wed, Jun 28, 2023 at 10:00:56AM +0800, Baolu Lu wrote:
> > If the driver created a SVA domain then the op should point to some
> > generic 'handle sva fault' function. There shouldn't be weird SVA
> > stuff in the core code.
> > 
> > The weird SVA stuff is really just a generic per-device workqueue
> > dispatcher, so if we think that is valuable then it should be
> > integrated into the iommu_domain (domain->ops->use_iopf_workqueue =
> > true for instance). Then it could route the fault through the
> > workqueue and still invoke domain->ops->iopf_handler.
> > 
> > The word "SVA" should not appear in any of this.
> 
> Yes. We should make it generic. The domain->use_iopf_workqueue flag
> denotes that the page faults of a fault group should be put together and
> then be handled and responded in a workqueue. Otherwise, the page fault
> is dispatched to domain->iopf_handler directly.

It might be better to have iopf_handler and
iopf_handler_work function pointers to distinguish to two cases.

> > Not sure what iommu_register_device_fault_handler() has to do with all
> > of this.. Setting up the dev_iommu stuff to allow for the workqueue
> > should happen dynamically during domain attach, ideally in the core
> > code before calling to the driver.
> 
> There are two pointers under struct dev_iommu for fault handling.
> 
> /**
>  * struct dev_iommu - Collection of per-device IOMMU data
>  *
>  * @fault_param: IOMMU detected device fault reporting data
>  * @iopf_param:  I/O Page Fault queue and data
> 
> [...]
> 
> struct dev_iommu {
>         struct mutex lock;
>         struct iommu_fault_param        *fault_param;
>         struct iopf_device_param        *iopf_param;
> 
> My understanding is that @fault_param is a place holder for generic
> things, while @iopf_param is workqueue specific.

Well, lets look

struct iommu_fault_param {
	iommu_dev_fault_handler_t handler;
	void *data;

These two make no sense now. handler is always iommu_queue_iopf. Given
our domain centric design we want the function pointer in the domain,
not in the device. So delete it.

	struct list_head faults;
	struct mutex lock;

Queue of unhandled/unacked faults? Seems sort of reasonable

> @iopf_param could be allocated on demand. (perhaps renaming it to a more
> meaningful one?) It happens before a domain with use_iopf_workqueue flag
> set attaches to a device. iopf_param keeps alive until device_release.

Yes

Do this for the iommu_fault_param as well, in fact, probably just put
the two things together in one allocation and allocate if we attach a
PRI using domain. I don't think we need to micro optimze further..
 
Jason