Re: [PATCH v6 20/27] cxl/extent: Process DCD events and realize region extents

From: Jonathan Cameron
Date: Thu Nov 07 2024 - 06:30:08 EST


On Tue, 05 Nov 2024 12:38:42 -0600
ira.weiny@xxxxxxxxx wrote:

> From: Navneet Singh <navneet.singh@xxxxxxxxx>
>
> A dynamic capacity device (DCD) sends events to signal the host for
> changes in the availability of Dynamic Capacity (DC) memory. These
> events contain extents describing a DPA range and meta data for memory
> to be added or removed. Events may be sent from the device at any time.
>
> Three types of events can be signaled, Add, Release, and Force Release.
>
> On add, the host may accept or reject the memory being offered. If no
> region exists, or the extent is invalid, the extent should be rejected.
> Add extent events may be grouped by a 'more' bit which indicates those
> extents should be processed as a group.
>
> On remove, the host can delay the response until the host is safely not
> using the memory. If no region exists the release can be sent
> immediately. The host may also release extents (or partial extents) at
> any time. Thus the 'more' bit grouping of release events is of less
> value and can be ignored in favor of sending multiple release capacity
> responses for groups of release events.
>
> Force removal is intended as a mechanism between the FM and the device
> and intended only when the host is unresponsive, out of sync, or
> otherwise broken. Purposely ignore force removal events.
>
> Regions are made up of one or more devices which may be surfacing memory
> to the host. Once all devices in a region have surfaced an extent the
> region can expose a corresponding extent for the user to consume.
> Without interleaving a device extent forms a 1:1 relationship with the
> region extent. Immediately surface a region extent upon getting a
> device extent.
>
> Per the specification the device is allowed to offer or remove extents
> at any time. However, anticipated use cases can expect extents to be
> offered, accepted, and removed in well defined chunks.
>
> Simplify extent tracking with the following restrictions.
>
> 1) Flag for removal any extent which overlaps a requested
> release range.
> 2) Refuse the offer of extents which overlap already accepted
> memory ranges.
> 3) Accept again a range which has already been accepted by the
> host. Eating duplicates serves three purposes. First, this
> simplifies the code if the device should get out of sync with
> the host. And it should be safe to acknowledge the extent
> again. Second, this simplifies the code to process existing
> extents if the extent list should change while the extent
> list is being read. Third, duplicates for a given region
> which are seen during a race between the hardware surfacing
> an extent and the cxl dax driver scanning for existing
> extents will be ignored.
>
> NOTE: Processing existing extents is done in a later patch.
>
> Management of the region extent devices must be synchronized with
> potential uses of the memory within the DAX layer. Create region extent
> devices as children of the cxl_dax_region device such that the DAX
> region driver can co-drive them and synchronize with the DAX layer.
> Synchronization and management is handled in a subsequent patch.
>
> Tag support within the DAX layer is not yet supported. To maintain
> compatibility legacy DAX/region processing only tags with a value of 0
> are allowed. This defines existing DAX devices as having a 0 tag which
> makes the most logical sense as a default.
>
> Process DCD events and create region devices.
>
> Signed-off-by: Navneet Singh <navneet.singh@xxxxxxxxx>
> Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>
> Co-developed-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
>
I gave this on v5, but meh here it is again (obviously subject to
Li Ming's fix which maybe can just get rolled in whilst applying)

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>