Re: [PATCH v5 01/15] cxl/region: Add region creation ABI

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Feb 17, 2022 at 9:19 AM Ben Widawsky <ben.widawsky@intel.com> wrote:
>
> Regions are created as a child of the decoder that encompasses an
> address space with constraints. Regions have a number of attributes that
> must be configured before the region can be activated.
>
> The ABI is not meant to be secure, but is meant to avoid accidental
> races. As a result, a buggy process may create a region by name that was
> allocated by a different process. However, multiple processes which are
> trying not to race with each other shouldn't need special
> synchronization to do so.
>
> // Allocate a new region name
> region=$(cat /sys/bus/cxl/devices/decoder0.0/create_region)
>
> // Create a new region by name
> while
> region=$(cat /sys/bus/cxl/devices/decoder0.0/create_region)
> ! echo $region > /sys/bus/cxl/devices/decoder0.0/create_region
> do true; done
>
> // Region now exists in sysfs
> stat -t /sys/bus/cxl/devices/decoder0.0/$region
>
> // Delete the region, and name
> echo $region > /sys/bus/cxl/devices/decoder0.0/delete_region
>
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>

Looking good, a few more fixes and cleanups identified below.

>
> ---
> Changes since v4:
> - Add the missed base attributes addition
>
> ---
>  Documentation/ABI/testing/sysfs-bus-cxl       |  23 ++
>  .../driver-api/cxl/memory-devices.rst         |  11 +
>  drivers/cxl/core/Makefile                     |   1 +
>  drivers/cxl/core/core.h                       |   3 +
>  drivers/cxl/core/port.c                       |  11 +
>  drivers/cxl/core/region.c                     | 213 ++++++++++++++++++
>  drivers/cxl/cxl.h                             |   5 +
>  drivers/cxl/region.h                          |  23 ++
>  tools/testing/cxl/Kbuild                      |   1 +
>  9 files changed, 291 insertions(+)
>  create mode 100644 drivers/cxl/core/region.c
>  create mode 100644 drivers/cxl/region.h
>
> diff --git a/Documentation/ABI/testing/sysfs-bus-cxl b/Documentation/ABI/testing/sysfs-bus-cxl
> index 7c2b846521f3..e5db45ea70ad 100644
> --- a/Documentation/ABI/testing/sysfs-bus-cxl
> +++ b/Documentation/ABI/testing/sysfs-bus-cxl
> @@ -163,3 +163,26 @@ Description:
>                 memory (type-3). The 'target_type' attribute indicates the
>                 current setting which may dynamically change based on what
>                 memory regions are activated in this decode hierarchy.
> +
> +What:          /sys/bus/cxl/devices/decoderX.Y/create_region
> +Date:          January, 2022
> +KernelVersion: v5.18
> +Contact:       linux-cxl@vger.kernel.org
> +Description:
> +               Write a value of the form 'regionX.Y:Z' to instantiate a new
> +               region within the decode range bounded by decoderX.Y. The value
> +               written must match the current value returned from reading this
> +               attribute. This behavior lets the kernel arbitrate racing
> +               attempts to create a region. The thread that fails to write
> +               loops and tries the next value. Regions must be created for root
> +               decoders, and must subsequently configured and bound to a region
> +               driver before they can be used.
> +
> +What:          /sys/bus/cxl/devices/decoderX.Y/delete_region
> +Date:          January, 2022
> +KernelVersion: v5.18
> +Contact:       linux-cxl@vger.kernel.org
> +Description:
> +               Deletes the named region.  The attribute expects a region in the
> +               form "regionX.Y:Z". The region's name, allocated by reading
> +               create_region, will also be released.
> diff --git a/Documentation/driver-api/cxl/memory-devices.rst b/Documentation/driver-api/cxl/memory-devices.rst
> index db476bb170b6..66ddc58a21b1 100644
> --- a/Documentation/driver-api/cxl/memory-devices.rst
> +++ b/Documentation/driver-api/cxl/memory-devices.rst
> @@ -362,6 +362,17 @@ CXL Core
>  .. kernel-doc:: drivers/cxl/core/mbox.c
>     :doc: cxl mbox
>
> +CXL Regions
> +-----------
> +.. kernel-doc:: drivers/cxl/region.h
> +   :identifiers:
> +
> +.. kernel-doc:: drivers/cxl/core/region.c
> +   :doc: cxl core region
> +
> +.. kernel-doc:: drivers/cxl/core/region.c
> +   :identifiers:
> +
>  External Interfaces
>  ===================
>
> diff --git a/drivers/cxl/core/Makefile b/drivers/cxl/core/Makefile
> index 6d37cd78b151..39ce8f2f2373 100644
> --- a/drivers/cxl/core/Makefile
> +++ b/drivers/cxl/core/Makefile
> @@ -4,6 +4,7 @@ obj-$(CONFIG_CXL_BUS) += cxl_core.o
>  ccflags-y += -I$(srctree)/drivers/cxl
>  cxl_core-y := port.o
>  cxl_core-y += pmem.o
> +cxl_core-y += region.o
>  cxl_core-y += regs.o
>  cxl_core-y += memdev.o
>  cxl_core-y += mbox.o
> diff --git a/drivers/cxl/core/core.h b/drivers/cxl/core/core.h
> index 1a50c0fc399c..adfd42370b28 100644
> --- a/drivers/cxl/core/core.h
> +++ b/drivers/cxl/core/core.h
> @@ -9,6 +9,9 @@ extern const struct device_type cxl_nvdimm_type;
>
>  extern struct attribute_group cxl_base_attribute_group;
>
> +extern struct device_attribute dev_attr_create_region;
> +extern struct device_attribute dev_attr_delete_region;
> +
>  struct cxl_send_command;
>  struct cxl_mem_query_commands;
>  int cxl_query_cmd(struct cxl_memdev *cxlmd,
> diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
> index 1e785a3affaa..860e91cae29b 100644
> --- a/drivers/cxl/core/port.c
> +++ b/drivers/cxl/core/port.c
> @@ -213,6 +213,8 @@ static struct attribute_group cxl_decoder_base_attribute_group = {
>  };
>
>  static struct attribute *cxl_decoder_root_attrs[] = {
> +       &dev_attr_create_region.attr,
> +       &dev_attr_delete_region.attr,
>         &dev_attr_cap_pmem.attr,
>         &dev_attr_cap_ram.attr,
>         &dev_attr_cap_type2.attr,
> @@ -270,6 +272,8 @@ static void cxl_decoder_release(struct device *dev)
>         struct cxl_decoder *cxld = to_cxl_decoder(dev);
>         struct cxl_port *port = to_cxl_port(dev->parent);
>
> +       ida_free(&cxld->region_ida, cxld->next_region_id);
> +       ida_destroy(&cxld->region_ida);
>         ida_free(&port->decoder_ida, cxld->id);
>         kfree(cxld);
>  }
> @@ -1244,6 +1248,13 @@ static struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port,
>         cxld->target_type = CXL_DECODER_EXPANDER;
>         cxld->platform_res = (struct resource)DEFINE_RES_MEM(0, 0);
>
> +       mutex_init(&cxld->id_lock);
> +       ida_init(&cxld->region_ida);
> +       rc = ida_alloc(&cxld->region_ida, GFP_KERNEL);
> +       if (rc < 0)
> +               goto err;
> +
> +       cxld->next_region_id = rc;
>         return cxld;
>  err:
>         kfree(cxld);
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> new file mode 100644
> index 000000000000..5576952e4aa1
> --- /dev/null
> +++ b/drivers/cxl/core/region.c
> @@ -0,0 +1,213 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright(c) 2022 Intel Corporation. All rights reserved. */
> +#include <linux/device.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/idr.h>
> +#include <region.h>
> +#include <cxl.h>
> +#include "core.h"
> +
> +/**
> + * DOC: cxl core region
> + *
> + * CXL Regions represent mapped memory capacity in system physical address
> + * space. Whereas the CXL Root Decoders identify the bounds of potential CXL
> + * Memory ranges, Regions represent the active mapped capacity by the HDM
> + * Decoder Capability structures throughout the Host Bridges, Switches, and
> + * Endpoints in the topology.
> + */
> +
> +static void cxl_region_release(struct device *dev);

Why forward declare this versus move cxl_region_type after the definition?

No other CXL object release functions are forward declared.

> +
> +static const struct device_type cxl_region_type = {
> +       .name = "cxl_region",
> +       .release = cxl_region_release,
> +};
> +
> +static struct cxl_region *to_cxl_region(struct device *dev)
> +{
> +       if (dev_WARN_ONCE(dev, dev->type != &cxl_region_type,
> +                         "not a cxl_region device\n"))
> +               return NULL;
> +
> +       return container_of(dev, struct cxl_region, dev);
> +}
> +
> +static struct cxl_region *cxl_region_alloc(struct cxl_decoder *cxld)
> +{
> +       struct cxl_region *cxlr;
> +       struct device *dev;
> +
> +       cxlr = kzalloc(sizeof(*cxlr), GFP_KERNEL);
> +       if (!cxlr)
> +               return ERR_PTR(-ENOMEM);
> +
> +       dev = &cxlr->dev;
> +       device_initialize(dev);
> +       dev->parent = &cxld->dev;
> +       device_set_pm_not_required(dev);
> +       dev->bus = &cxl_bus_type;
> +       dev->type = &cxl_region_type;
> +
> +       return cxlr;
> +}
> +
> +static void unregister_region(void *_cxlr)
> +{
> +       struct cxl_region *cxlr = _cxlr;
> +
> +       if (!test_and_set_bit(REGION_DEAD, &cxlr->flags))
> +               device_unregister(&cxlr->dev);

I thought REGION_DEAD was needed to prevent double
devm_release_action(), not double unregister?

> +}
> +
> +/**
> + * devm_cxl_add_region - Adds a region to a decoder
> + * @cxld: Parent decoder.
> + * @cxlr: Region to be added to the decoder.
> + *
> + * This is the second step of region initialization. Regions exist within an
> + * address space which is mapped by a @cxld. That @cxld must be a root decoder,
> + * and it enforces constraints upon the region as it is configured.
> + *
> + * Return: 0 if the region was added to the @cxld, else returns negative error
> + * code. The region will be named "regionX.Y.Z" where X is the port, Y is the
> + * decoder id, and Z is the region number.
> + */
> +static struct cxl_region *devm_cxl_add_region(struct cxl_decoder *cxld)
> +{
> +       struct cxl_port *port = to_cxl_port(cxld->dev.parent);
> +       struct cxl_region *cxlr;
> +       struct device *dev;
> +       int rc;
> +
> +       cxlr = cxl_region_alloc(cxld);
> +       if (IS_ERR(cxlr))
> +               return cxlr;
> +
> +       dev = &cxlr->dev;
> +
> +       cxlr->id = cxld->next_region_id;
> +       rc = dev_set_name(dev, "region%d.%d:%d", port->id, cxld->id, cxlr->id);
> +       if (rc)
> +               goto err_out;
> +
> +       /* affirm that release will have access to the decoder's region ida  */
> +       get_device(&cxld->dev);
> +
> +       rc = device_add(dev);
> +       if (!rc)
> +               rc = devm_add_action_or_reset(port->uport, unregister_region,
> +                                             cxlr);
> +       if (rc)
> +               goto err_out;

All the other usages in device_add() in the subsystem follow the style of:

rc = device_add(dev);
if (rc)
    goto err;

...any reason to be unique here and indent the success case?


> +
> +       return cxlr;
> +
> +err_out:
> +       put_device(dev);
> +       kfree(cxlr);

This is a double-free of cxlr;

> +       return ERR_PTR(rc);
> +}
> +
> +static ssize_t create_region_show(struct device *dev,
> +                                 struct device_attribute *attr, char *buf)
> +{
> +       struct cxl_port *port = to_cxl_port(dev->parent);
> +       struct cxl_decoder *cxld = to_cxl_decoder(dev);
> +
> +       return sysfs_emit(buf, "region%d.%d:%d\n", port->id, cxld->id,
> +                         cxld->next_region_id);
> +}
> +
> +static ssize_t create_region_store(struct device *dev,
> +                                  struct device_attribute *attr,
> +                                  const char *buf, size_t len)
> +{
> +       struct cxl_port *port = to_cxl_port(dev->parent);
> +       struct cxl_decoder *cxld = to_cxl_decoder(dev);
> +       struct cxl_region *cxlr;
> +       int d, p, r, rc = 0;
> +
> +       if (sscanf(buf, "region%d.%d:%d", &p, &d, &r) != 3)
> +               return -EINVAL;
> +
> +       if (port->id != p || cxld->id != d)
> +               return -EINVAL;
> +
> +       rc = mutex_lock_interruptible(&cxld->id_lock);
> +       if (rc)
> +               return rc;
> +
> +       if (cxld->next_region_id != r) {
> +               rc = -EINVAL;
> +               goto out;
> +       }
> +
> +       rc = ida_alloc(&cxld->region_ida, GFP_KERNEL);
> +       if (rc < 0) {
> +               dev_dbg(dev, "Failed to get next cached id (%d)\n", rc);
> +               goto out;
> +       }
> +
> +       cxlr = devm_cxl_add_region(cxld);
> +       if (IS_ERR(cxlr)) {
> +               rc = PTR_ERR(cxlr);
> +               goto out;
> +       }
> +
> +       cxld->next_region_id = rc;

This looks like a leak in the case when devm_cxl_add_region() fails,
so just move it above that call.

> +       dev_dbg(dev, "Created %s\n", dev_name(&cxlr->dev));
> +
> +out:
> +       mutex_unlock(&cxld->id_lock);
> +       return rc ? rc : len;

if (rc)
    return rc;
return len;

> +}
> +DEVICE_ATTR_RW(create_region);
> +
> +static struct cxl_region *cxl_find_region_by_name(struct cxl_decoder *cxld,
> +                                                 const char *name)
> +{
> +       struct device *region_dev;
> +
> +       region_dev = device_find_child_by_name(&cxld->dev, name);
> +       if (!region_dev)
> +               return ERR_PTR(-ENOENT);
> +
> +       return to_cxl_region(region_dev);
> +}
> +
> +static ssize_t delete_region_store(struct device *dev,
> +                                  struct device_attribute *attr,
> +                                  const char *buf, size_t len)
> +{
> +       struct cxl_port *port = to_cxl_port(dev->parent);
> +       struct cxl_decoder *cxld = to_cxl_decoder(dev);
> +       struct cxl_region *cxlr;
> +
> +       cxlr = cxl_find_region_by_name(cxld, buf);
> +       if (IS_ERR(cxlr))
> +               return PTR_ERR(cxlr);
> +
> +       /* After this, the region is no longer a child of the decoder. */
> +       devm_release_action(port->uport, unregister_region, cxlr);

This may trigger a WARN in the case where 2 threads race to trigger
the release action. I think the DEAD check is needed to gate this
call, not device_unregister().

> +
> +       /* Release is likely called here, so cxlr is not safe to reference. */

This is always the case with any put_device(), so no need for this comment.

> +       put_device(&cxlr->dev);
> +       cxlr = NULL;

This NULL assignment has no value.

> +
> +       dev_dbg(dev, "Deleted %s\n", buf);

Not sure a debug statement is needed for something userspace can
directly view itself with the result code from the sysfs write.

> +       return len;
> +}
> +DEVICE_ATTR_WO(delete_region);
> +
> +static void cxl_region_release(struct device *dev)
> +{
> +       struct cxl_decoder *cxld = to_cxl_decoder(dev->parent);
> +       struct cxl_region *cxlr = to_cxl_region(dev);
> +
> +       dev_dbg(&cxld->dev, "Releasing %s\n", dev_name(dev));
> +       ida_free(&cxld->region_ida, cxlr->id);
> +       kfree(cxlr);
> +       put_device(&cxld->dev);
> +}
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index b4047a310340..d5397f7dfcf4 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -221,6 +221,8 @@ enum cxl_decoder_type {
>   * @target_type: accelerator vs expander (type2 vs type3) selector
>   * @flags: memory type capabilities and locking
>   * @target_lock: coordinate coherent reads of the target list
> + * @region_ida: allocator for region ids.
> + * @next_region_id: Cached region id for next region.
>   * @nr_targets: number of elements in @target
>   * @target: active ordered target list in current decoder configuration
>   */
> @@ -236,6 +238,9 @@ struct cxl_decoder {
>         enum cxl_decoder_type target_type;
>         unsigned long flags;
>         seqlock_t target_lock;
> +       struct mutex id_lock;
> +       struct ida region_ida;
> +       int next_region_id;
>         int nr_targets;
>         struct cxl_dport *target[];
>  };
> diff --git a/drivers/cxl/region.h b/drivers/cxl/region.h
> new file mode 100644
> index 000000000000..0016f83bbdfd
> --- /dev/null
> +++ b/drivers/cxl/region.h
> @@ -0,0 +1,23 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/* Copyright(c) 2021 Intel Corporation. */
> +#ifndef __CXL_REGION_H__
> +#define __CXL_REGION_H__
> +
> +#include <linux/uuid.h>
> +
> +#include "cxl.h"
> +
> +/**
> + * struct cxl_region - CXL region
> + * @dev: This region's device.
> + * @id: This region's id. Id is globally unique across all regions.
> + * @flags: Flags representing the current state of the region.
> + */
> +struct cxl_region {
> +       struct device dev;
> +       int id;
> +       unsigned long flags;
> +#define REGION_DEAD 0
> +};
> +
> +#endif
> diff --git a/tools/testing/cxl/Kbuild b/tools/testing/cxl/Kbuild
> index 82e49ab0937d..3fe6d34e6d59 100644
> --- a/tools/testing/cxl/Kbuild
> +++ b/tools/testing/cxl/Kbuild
> @@ -46,6 +46,7 @@ cxl_core-y += $(CXL_CORE_SRC)/memdev.o
>  cxl_core-y += $(CXL_CORE_SRC)/mbox.o
>  cxl_core-y += $(CXL_CORE_SRC)/pci.o
>  cxl_core-y += $(CXL_CORE_SRC)/hdm.o
> +cxl_core-y += $(CXL_CORE_SRC)/region.o
>  cxl_core-y += config_check.o
>
>  obj-m += test/
> --
> 2.35.1
>