KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 177 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-dax b/Documentation/ABI/testing/sysfs-bus-dax
new file mode 100644
index 000000000..60703fffa
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-dax
@@ -0,0 +1,177 @@
+What:		/sys/bus/dax/devices/daxX.Y/align
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) Provides a way to specify an alignment for a dax device.
+		Values allowed are constrained by the physical address ranges
+		that back the dax device, and also by arch requirements.
+
+What:		/sys/bus/dax/devices/daxX.Y/mapping
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(WO) Provides a way to allocate a mapping range under a dax
+		device. Specified in the format <start>-<end>.
+
+What:		/sys/bus/dax/devices/daxX.Y/mapping[0..N]/start
+What:		/sys/bus/dax/devices/daxX.Y/mapping[0..N]/end
+What:		/sys/bus/dax/devices/daxX.Y/mapping[0..N]/page_offset
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) A dax device may have multiple constituent discontiguous
+		address ranges. These are represented by the different
+		'mappingX' subdirectories. The 'start' attribute indicates the
+		start physical address for the given range. The 'end' attribute
+		indicates the end physical address for the given range. The
+		'page_offset' attribute indicates the offset of the current
+		range in the dax device.
+
+What:		/sys/bus/dax/devices/daxX.Y/resource
+Date:		June, 2019
+KernelVersion:	v5.3
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The resource attribute indicates the starting physical
+		address of a dax device. In case of a device with multiple
+		constituent ranges, it indicates the starting address of the
+		first range.
+
+What:		/sys/bus/dax/devices/daxX.Y/size
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) The size attribute indicates the total size of a dax
+		device. For creating subdivided dax devices, or for resizing
+		an existing device, the new size can be written to this as
+		part of the reconfiguration process.
+
+What:		/sys/bus/dax/devices/daxX.Y/numa_node
+Date:		November, 2019
+KernelVersion:	v5.5
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) If NUMA is enabled and the platform has affinitized the
+		backing device for this dax device, emit the CPU node
+		affinity for this device.
+
+What:		/sys/bus/dax/devices/daxX.Y/target_node
+Date:		February, 2019
+KernelVersion:	v5.1
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The target-node attribute is the Linux numa-node that a
+		device-dax instance may create when it is online. Prior to
+		being online the device's 'numa_node' property reflects the
+		closest online cpu node which is the typical expectation of a
+		device 'numa_node'. Once it is online it becomes its own
+		distinct numa node.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/available_size
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The available_size attribute tracks available dax region
+		capacity. This only applies to volatile hmem devices, not pmem
+		devices, since pmem devices are defined by nvdimm namespace
+		boundaries.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/size
+Date:		July, 2017
+KernelVersion:	v5.1
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The size attribute indicates the size of a given dax region
+		in bytes.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/align
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The align attribute indicates alignment of the dax region.
+		Changes on align may not always be valid, when say certain
+		mappings were created with 2M and then we switch to 1G. This
+		validates all ranges against the new value being attempted, post
+		resizing.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/seed
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The seed device is a concept for dynamic dax regions to be
+		able to split the region amongst multiple sub-instances.  The
+		seed device, similar to libnvdimm seed devices, is a device
+		that starts with zero capacity allocated and unbound to a
+		driver.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/create
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) The create interface to the dax region provides a way to
+		create a new unconfigured dax device under the given region, which
+		can then be configured (with a size etc.) and then probed.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/delete
+Date:		October, 2020
+KernelVersion:	v5.10
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(WO) The delete interface for a dax region provides for deletion
+		of any 0-sized and idle dax devices.
+
+What:		$(readlink -f /sys/bus/dax/devices/daxX.Y)/../dax_region/id
+Date:		July, 2017
+KernelVersion:	v5.1
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RO) The id attribute indicates the region id of a dax region.
+
+What:		/sys/bus/dax/devices/daxX.Y/memmap_on_memory
+Date:		January, 2024
+KernelVersion:	v6.8
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) Control the memmap_on_memory setting if the dax device
+		were to be hotplugged as system memory. This determines whether
+		the 'altmap' for the hotplugged memory will be placed on the
+		device being hotplugged (memmap_on_memory=1) or if it will be
+		placed on regular memory (memmap_on_memory=0). This attribute
+		must be set before the device is handed over to the 'kmem'
+		driver (i.e.  hotplugged into system-ram). Additionally, this
+		depends on CONFIG_MHP_MEMMAP_ON_MEMORY, and a globally enabled
+		memmap_on_memory parameter for memory_hotplug. This is
+		typically set on the kernel command line -
+		memory_hotplug.memmap_on_memory set to 'true' or 'force'."
+
+What:		/sys/bus/dax/devices/daxX.Y/state
+Contact:	nvdimm@lists.linux.dev
+Description:
+		(RW) Controls the state of the memory region.
+		Applies to all memory blocks associated with the device.
+		Only applies to dax_kmem devices.
+
+		Reading returns the current state; the writable states mirror
+		the per-block /sys/devices/system/memory/memoryX/state ABI::
+
+		  "unplugged": memory blocks are not present
+		  "online": memory is online, zone chosen by the kernel
+		  "online_kernel": memory is online in ZONE_NORMAL
+		  "online_movable": memory is online in ZONE_MOVABLE
+
+		"offline" (memory blocks are present but offline) may also be
+		reported - this happens when the device is bound while the
+		auto_online_blocks policy is "offline".  It cannot be written,
+		as it's not useful and creates device destruction races.
+
+		A device can only be onlined from the "unplugged" state, so a
+		device must be returned to "unplugged" before it can be onlined
+		into a different state.