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, 90 insertions, 0 deletions

diff --git a/Documentation/driver-api/mtd/spi-intel.rst b/Documentation/driver-api/mtd/spi-intel.rst
new file mode 100644
index 000000000..df854f20e
--- /dev/null
+++ b/Documentation/driver-api/mtd/spi-intel.rst
@@ -0,0 +1,90 @@
+==============================
+Upgrading BIOS using spi-intel
+==============================
+
+Many Intel CPUs like Baytrail and Braswell include SPI serial flash host
+controller which is used to hold BIOS and other platform specific data.
+Since contents of the SPI serial flash is crucial for machine to function,
+it is typically protected by different hardware protection mechanisms to
+avoid accidental (or on purpose) overwrite of the content.
+
+Not all manufacturers protect the SPI serial flash, mainly because it
+allows upgrading the BIOS image directly from an OS.
+
+The spi-intel driver makes it possible to read and write the SPI serial
+flash, if certain protection bits are not set and locked. If it finds
+any of them set, the whole MTD device is made read-only to prevent
+partial overwrites. By default the driver exposes SPI serial flash
+contents as read-only but it can be changed from kernel command line,
+passing "spi_intel.writeable=1".
+
+Please keep in mind that overwriting the BIOS image on SPI serial flash
+might render the machine unbootable and requires special equipment like
+Dediprog to revive. You have been warned!
+
+Below are the steps how to upgrade MinnowBoard MAX BIOS directly from
+Linux.
+
+ 1) Download and extract the latest Minnowboard MAX BIOS SPI image
+    [1]. At the time writing this the latest image is v92.
+
+ 2) Install mtd-utils package [2]. We need this in order to erase the SPI
+    serial flash. Distros like Debian and Fedora have this prepackaged with
+    name "mtd-utils".
+
+ 3) Add "spi_intel.writeable=1" to the kernel command line and reboot
+    the board (you can also reload the driver passing "writeable=1" as
+    module parameter to modprobe).
+
+ 4) Once the board is up and running again, find the right MTD partition
+    (it is named as "BIOS")::
+
+	# cat /proc/mtd
+	dev:    size   erasesize  name
+	mtd0: 00800000 00001000 "BIOS"
+
+    So here it will be /dev/mtd0 but it may vary.
+
+ 5) Make backup of the existing image first::
+
+	# dd if=/dev/mtd0ro of=bios.bak
+	16384+0 records in
+	16384+0 records out
+	8388608 bytes (8.4 MB) copied, 10.0269 s, 837 kB/s
+
+ 6) Verify the backup::
+
+	# sha1sum /dev/mtd0ro bios.bak
+	fdbb011920572ca6c991377c4b418a0502668b73  /dev/mtd0ro
+	fdbb011920572ca6c991377c4b418a0502668b73  bios.bak
+
+    The SHA1 sums must match. Otherwise do not continue any further!
+
+ 7) Erase the SPI serial flash. After this step, do not reboot the
+    board! Otherwise it will not start anymore::
+
+	# flash_erase /dev/mtd0 0 0
+	Erasing 4 Kibyte @ 7ff000 -- 100 % complete
+
+ 8) Once completed without errors you can write the new BIOS image::
+
+    # dd if=MNW2MAX1.X64.0092.R01.1605221712.bin of=/dev/mtd0
+
+ 9) Verify that the new content of the SPI serial flash matches the new
+    BIOS image::
+
+	# sha1sum /dev/mtd0ro MNW2MAX1.X64.0092.R01.1605221712.bin
+	9b4df9e4be2057fceec3a5529ec3d950836c87a2  /dev/mtd0ro
+	9b4df9e4be2057fceec3a5529ec3d950836c87a2 MNW2MAX1.X64.0092.R01.1605221712.bin
+
+    The SHA1 sums should match.
+
+ 10) Now you can reboot your board and observe the new BIOS starting up
+     properly.
+
+References
+----------
+
+[1] https://firmware.intel.com/sites/default/files/MinnowBoard%2EMAX_%2EX64%2E92%2ER01%2Ezip
+
+[2] http://www.linux-mtd.infradead.org/