kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 31 insertions, 0 deletions

diff --git a/Documentation/admin-guide/sysctl/xen.rst b/Documentation/admin-guide/sysctl/xen.rst
new file mode 100644
index 000000000..6c5edc3e5
--- /dev/null
+++ b/Documentation/admin-guide/sysctl/xen.rst
@@ -0,0 +1,31 @@
+===============
+/proc/sys/xen/
+===============
+
+Copyright (c) 2026, Shubham Chakraborty <chakrabortyshubham66@gmail.com>
+
+For general info and legal blurb, please look in
+Documentation/admin-guide/sysctl/index.rst.
+
+------------------------------------------------------------------------------
+
+These files show up in ``/proc/sys/xen/``, depending on the
+kernel configuration:
+
+.. contents:: :local:
+
+balloon/hotplug_unpopulated
+===========================
+
+This flag controls whether unpopulated memory ranges are automatically
+hotplugged as system RAM.
+
+- ``0``: Unpopulated ranges are not hotplugged (default).
+- ``1``: Unpopulated ranges are automatically hotplugged.
+
+When enabled, the Xen balloon driver will add memory regions that are
+marked as unpopulated in the Xen memory map to the system as usable RAM.
+This allows for dynamic memory expansion in Xen guest domains.
+
+This option is only available when the kernel is built with
+``CONFIG_XEN_BALLOON_MEMORY_HOTPLUG`` enabled.