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, 86 insertions, 0 deletions

diff --git a/Documentation/fb/internals.rst b/Documentation/fb/internals.rst
new file mode 100644
index 000000000..696b50aa7
--- /dev/null
+++ b/Documentation/fb/internals.rst
@@ -0,0 +1,86 @@
+=============================
+Frame Buffer device internals
+=============================
+
+This is a first start for some documentation about frame buffer device
+internals.
+
+Authors:
+
+- Geert Uytterhoeven <geert@linux-m68k.org>, 21 July 1998
+- James Simmons <jsimmons@user.sf.net>, Nov 26 2002
+
+--------------------------------------------------------------------------------
+
+Structures used by the frame buffer device API
+==============================================
+
+The following structures play a role in the game of frame buffer devices. They
+are defined in <linux/fb.h>.
+
+1. Outside the kernel (user space)
+
+  - struct fb_fix_screeninfo
+
+    Device independent unchangeable information about a frame buffer device and
+    a specific video mode. This can be obtained using the FBIOGET_FSCREENINFO
+    ioctl.
+
+  - struct fb_var_screeninfo
+
+    Device independent changeable information about a frame buffer device and a
+    specific video mode. This can be obtained using the FBIOGET_VSCREENINFO
+    ioctl, and updated with the FBIOPUT_VSCREENINFO ioctl. If you want to pan
+    the screen only, you can use the FBIOPAN_DISPLAY ioctl.
+
+  - struct fb_cmap
+
+    Device independent colormap information. You can get and set the colormap
+    using the FBIOGETCMAP and FBIOPUTCMAP ioctls.
+
+
+2. Inside the kernel
+
+  - struct fb_info
+
+    Generic information, API and low level information about a specific frame
+    buffer device instance (slot number, board address, ...).
+
+  - struct `par`
+
+    Device dependent information that uniquely defines the video mode for this
+    particular piece of hardware.
+
+
+Visuals used by the frame buffer device API
+===========================================
+
+
+Monochrome (FB_VISUAL_MONO01 and FB_VISUAL_MONO10)
+--------------------------------------------------
+Each pixel is either black or white.
+
+
+Pseudo color (FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR)
+---------------------------------------------------------------------
+The whole pixel value is fed through a programmable lookup table that has one
+color (including red, green, and blue intensities) for each possible pixel
+value, and that color is displayed.
+
+
+True color (FB_VISUAL_TRUECOLOR)
+--------------------------------
+The pixel value is broken up into red, green, and blue fields.
+
+
+Direct color (FB_VISUAL_DIRECTCOLOR)
+------------------------------------
+The pixel value is broken up into red, green, and blue fields, each of which
+are looked up in separate red, green, and blue lookup tables.
+
+
+Grayscale displays
+------------------
+Grayscale and static grayscale are special variants of pseudo color and static
+pseudo color, where the red, green and blue components are always equal to
+each other.