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, 133 insertions, 0 deletions

diff --git a/Documentation/admin-guide/device-mapper/dm-init.rst b/Documentation/admin-guide/device-mapper/dm-init.rst
new file mode 100644
index 000000000..981d6a907
--- /dev/null
+++ b/Documentation/admin-guide/device-mapper/dm-init.rst
@@ -0,0 +1,133 @@
+================================
+Early creation of mapped devices
+================================
+
+It is possible to configure a device-mapper device to act as the root device for
+your system in two ways.
+
+The first is to build an initial ramdisk which boots to a minimal userspace
+which configures the device, then pivot_root(8) in to it.
+
+The second is to create one or more device-mappers using the module parameter
+"dm-mod.create=" through the kernel boot command line argument.
+
+The format is specified as a string of data separated by commas and optionally
+semi-colons, where:
+
+ - a comma is used to separate fields like name, uuid, flags and table
+   (specifies one device)
+ - a semi-colon is used to separate devices.
+
+So the format will look like this::
+
+ dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
+
+Where::
+
+	<name>		::= The device name.
+	<uuid>		::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | ""
+	<minor>		::= The device minor number | ""
+	<flags>		::= "ro" | "rw"
+	<table>		::= <start_sector> <num_sectors> <target_type> <target_args>
+	<target_type>	::= "verity" | "linear" | ... (see list below)
+
+The dm line should be equivalent to the one used by the dmsetup tool with the
+`--concise` argument.
+
+Target types
+============
+
+Not all target types are available as there are serious risks in allowing
+activation of certain DM targets without first using userspace tools to check
+the validity of associated metadata.
+
+======================= =======================================================
+`cache`			constrained, userspace should verify cache device
+`crypt`			allowed
+`delay`			allowed
+`era`			constrained, userspace should verify metadata device
+`flakey`		constrained, meant for test
+`linear`		allowed
+`log-writes`		constrained, userspace should verify metadata device
+`mirror`		constrained, userspace should verify main/mirror device
+`raid`			constrained, userspace should verify metadata device
+`snapshot`		constrained, userspace should verify src/dst device
+`snapshot-origin`	allowed
+`snapshot-merge`	constrained, userspace should verify src/dst device
+`striped`		allowed
+`switch`		constrained, userspace should verify dev path
+`thin`			constrained, requires dm target message from userspace
+`thin-pool`		constrained, requires dm target message from userspace
+`verity`		allowed
+`writecache`		constrained, userspace should verify cache device
+`zero`			constrained, not meant for rootfs
+======================= =======================================================
+
+If the target is not listed above, it is constrained by default (not tested).
+
+Examples
+========
+An example of booting to a linear array made up of user-mode linux block
+devices::
+
+  dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0
+
+This will boot to a rw dm-linear target of 8192 sectors split across two block
+devices identified by their major:minor numbers.  After boot, udev will rename
+this target to /dev/mapper/lroot (depending on the rules). No uuid was assigned.
+
+An example of multiple device-mappers, with the dm-mod.create="..." contents
+is shown here split on multiple lines for readability::
+
+  dm-linear,,1,rw,
+    0 32768 linear 8:1 0,
+    32768 1024000 linear 8:2 0;
+  dm-verity,,3,ro,
+    0 1638400 verity 1 /dev/sdc1 /dev/sdc2 4096 4096 204800 1 sha256
+    ac87db56303c9c1da433d7209b5a6ef3e4779df141200cbd7c157dcb8dd89c42
+    5ebfe87f7df3235b80a117ebc4078e44f55045487ad4a96581d1adb564615b51
+
+Other examples (per target):
+
+"crypt"::
+
+  dm-crypt,,8,ro,
+    0 1048576 crypt aes-xts-plain64
+    babebabebabebabebabebabebabebabebabebabebabebabebabebabebabebabe 0
+    /dev/sda 0 1 allow_discards
+
+"delay"::
+
+  dm-delay,,4,ro,0 409600 delay /dev/sda1 0 500
+
+"linear"::
+
+  dm-linear,,,rw,
+    0 32768 linear /dev/sda1 0,
+    32768 1024000 linear /dev/sda2 0,
+    1056768 204800 linear /dev/sda3 0,
+    1261568 512000 linear /dev/sda4 0
+
+"snapshot-origin"::
+
+  dm-snap-orig,,4,ro,0 409600 snapshot-origin 8:2
+
+"striped"::
+
+  dm-striped,,4,ro,0 1638400 striped 4 4096
+  /dev/sda1 0 /dev/sda2 0 /dev/sda3 0 /dev/sda4 0
+
+"verity"::
+
+  dm-verity,,4,ro,
+    0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
+    fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
+    51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584
+
+For setups using device-mapper on top of asynchronously probed block
+devices (MMC, USB, ..), it may be necessary to tell dm-init to
+explicitly wait for them to become available before setting up the
+device-mapper tables. This can be done with the "dm-mod.waitfor="
+module parameter, which takes a list of devices to wait for::
+
+  dm-mod.waitfor=<device1>[,..,<deviceN>]