linux-kernel - RE: [PATCH v5 13/23] iommu: introduce device fault report API

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Date:   Thu, 17 May 2018 23:22:43 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "David Woodhouse" <dwmw2@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Jean Delvare <khali@...ux-fr.org>,
        "Christoph Hellwig" <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: RE: [PATCH v5 13/23] iommu: introduce device fault report API

> From: Jacob Pan [mailto:jacob.jun.pan@...ux.intel.com]
> Sent: Thursday, May 17, 2018 11:59 PM
> On Thu, 17 May 2018 11:41:56 +0000
> "Liu, Yi L" <yi.l.liu@...el.com> wrote:
> 
> > > +int iommu_report_device_fault(struct device *dev, struct
> > > +iommu_fault_event *evt) {
> > > +	int ret = 0;
> > > +	struct iommu_fault_event *evt_pending;
> > > +	struct iommu_fault_param *fparam;
> > > +
> > > +	/* iommu_param is allocated when device is added to group
> > > */
> > > +	if (!dev->iommu_param | !evt)
> > > +		return -EINVAL;
> > > +	/* we only report device fault if there is a handler
> > > registered */
> > > +	mutex_lock(&dev->iommu_param->lock);
> > > +	if (!dev->iommu_param->fault_param ||
> > > +		!dev->iommu_param->fault_param->handler) {
> > > +		ret = -EINVAL;
> > > +		goto done_unlock;
> > > +	}
> > > +	fparam = dev->iommu_param->fault_param;
> > > +	if (evt->type == IOMMU_FAULT_PAGE_REQ && evt->last_req) {
> > > +		evt_pending = kmemdup(evt, sizeof(struct
> > > iommu_fault_event),
> > > +				GFP_KERNEL);
> > > +		if (!evt_pending) {
> > > +			ret = -ENOMEM;
> > > +			goto done_unlock;
> > > +		}
> > > +		mutex_lock(&fparam->lock);
> > > +		list_add_tail(&evt_pending->list,
> > > &fparam->faults);
> >
> > I may missed it. Here only see list add, how about removing? Who would
> > remove entry from the fault list?
> >
> deletion of the pending event is in page response function (int
> iommu_page_response), once iommu driver finds a matching response for the
> pending request, it will delete the pending event.
> 
> if the response never came, right now we don't delete it, just gives warning.

Got it.

> 
> > > +		mutex_unlock(&fparam->lock);
> > > +	}
> > > +	ret = fparam->handler(evt, fparam->data);
> >
> > I remember you mentioned there will be a queue to store the faults.
> > Is it in the fparam->faults list? Or there is no such queue?
> There are two use cases:
> case A: guest SVA, PRQ events are reported outside IOMMU subsystem,
> 	e.g. vfio
> case B: in-kernel
> 
> The io page fault queuing is Jean's patchset, mostly for case B (in-kernel IO page
> fault handling). I will convert intel-svm to Jean's io page fault mechanism so that we
> can also have parallel and out of order queuing of PRQ. I still need some time to
> evaluate intel specific needs such as streaming page request/response.
> 
> For case A, there is no queuing in host IOMMU driver. My understanding of the flow
> is as the following:
> 1. host IOMMU receives PRQ
> 2. host IOMMU driver reports PRQ fault event to registered called, i.e.
> vfio
> 3. VFIO reports fault event to QEMU
> 4. QEMU injects PRQ to guest
> 5. Guest IOMMU driver receives PRQ in IRQ 6. Guest IOMMU driver queue PRQ by
> groups, PASID.

Correct.

> So as long as in-kernel PRQ handling can do queuing, there is no need for queuing in
> the host reporting path.

Will it affect current interface? Here the handler only get an "evt" per a PRQ IRQ. And I suppose
vfio needs not rely on host iommu queuing?

Thanks,
Yi Liu