linux-kernel - Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report

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Message-ID: <20250120181854.GQ5556@nvidia.com>
Date: Mon, 20 Jan 2025 14:18:54 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
	eric.auger@...hat.com, jean-philippe@...aro.org, mdf@...nel.org,
	mshavit@...gle.com, shameerali.kolothum.thodi@...wei.com,
	smostafa@...gle.com, ddutile@...hat.com, yi.l.liu@...el.com,
	patches@...ts.linux.dev
Subject: Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event
 helper

On Fri, Jan 17, 2025 at 02:11:15PM -0800, Nicolin Chen wrote:
> +/**
> + * struct iommufd_vevent_header - Virtual Event Header for a vEVENTQ Status
> + * @state: One of enum iommu_veventq_state

I'd probably just make this a flag with overflow as the only current flag?

> + * @counter: A counter reflecting the state of the vEVENTQ

> + * ----------------------------------------------------------------------------
> + * | @state                       | @counter                                  |
> + * ----------------------------------------------------------------------------
> + * | IOMMU_VEVENTQ_STATE_OK       | number of readable vEVENTs in the vEVENTQ |
> + * | IOMMU_VEVENTQ_STATE_OVERFLOW | number of missed vEVENTs since overflow   |
> + * ----------------------------------------------------------------------------

When I said counter I literally ment a counter of the number of events
that were sent into the queue. So if events are dropped there is a
trivial gap in the count. Very easy to implement

IOMMU_VEVENTQ_STATE_OVERFLOW with a 0 length event is seen if events
have been lost and no subsequent events are present. It exists to
ensure timely delivery of the overflow event to userspace. counter
will be the sequence number of the next successful event.

If events are lost in the middle of the queue then flags will remain 0
but counter will become non-montonic. A counter delta > 1 indicates
that many events have been lost.

Jason