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, 136 insertions, 0 deletions

diff --git a/drivers/tty/hvc/Kconfig b/drivers/tty/hvc/Kconfig
new file mode 100644
index 000000000..c2a4e88b3
--- /dev/null
+++ b/drivers/tty/hvc/Kconfig
@@ -0,0 +1,136 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config HVC_DRIVER
+	bool
+	help
+	  Generic "hypervisor virtual console" infrastructure for various
+	  hypervisors (pSeries, iSeries, Xen).
+	  It will automatically be selected if one of the back-end console drivers
+	  is selected.
+
+config HVC_IRQ
+	bool
+
+config HVC_CONSOLE
+	bool "pSeries Hypervisor Virtual Console support"
+	depends on PPC_PSERIES
+	select HVC_DRIVER
+	select HVC_IRQ
+	help
+	  pSeries machines when partitioned support a hypervisor virtual
+	  console. This driver allows each pSeries partition to have a console
+	  which is accessed via the HMC.
+
+config HVC_OLD_HVSI
+	bool "Old driver for pSeries serial port (/dev/hvsi*)"
+	depends on HVC_CONSOLE
+
+config HVC_OPAL
+	bool "OPAL Console support"
+	depends on PPC_POWERNV
+	select HVC_DRIVER
+	select HVC_IRQ
+	default y
+	help
+	  PowerNV machines running under OPAL need that driver to get a console
+
+config HVC_RTAS
+	bool "IBM RTAS Console support"
+	depends on PPC_RTAS
+	select HVC_DRIVER
+	help
+	  IBM Console device driver which makes use of RTAS
+
+config HVC_IUCV
+	bool "z/VM IUCV Hypervisor console support (VM only)"
+	depends on S390 && NET
+	select HVC_DRIVER
+	select IUCV
+	default y
+	help
+	  This driver provides a Hypervisor console (HVC) back-end to access
+	  a Linux (console) terminal via a z/VM IUCV communication path.
+
+config HVC_XEN
+	bool "Xen Hypervisor Console support"
+	depends on XEN
+	select HVC_DRIVER
+	select HVC_IRQ
+	default y
+	help
+	  Xen virtual console device driver
+
+config HVC_XEN_FRONTEND
+	bool "Xen Hypervisor Multiple Consoles support"
+	depends on HVC_XEN
+	select XEN_XENBUS_FRONTEND
+	default y
+	help
+	  Xen driver for secondary virtual consoles
+
+config HVC_UDBG
+	bool "udbg based fake hypervisor console"
+	depends on PPC
+	select HVC_DRIVER
+	help
+	  This is meant to be used during HW bring up or debugging when
+	  no other console mechanism exist but udbg, to get you a quick
+	  console for userspace. Do NOT enable in production kernels.
+
+config HVC_DCC
+	bool "ARM JTAG DCC console"
+	depends on ARM || ARM64
+	select HVC_DRIVER
+	select SERIAL_CORE_CONSOLE
+	help
+	  This console uses the JTAG DCC on ARM to create a console under the HVC
+	  driver. This console is used through a JTAG only on ARM. If you don't have
+	  a JTAG then you probably don't want this option.
+
+config HVC_DCC_SERIALIZE_SMP
+	bool "Use DCC only on CPU core 0"
+	depends on SMP && HVC_DCC
+	help
+	  This is a DEBUG option to serialize all console input and output to CPU 0.
+	  Some external debuggers, do not handle reads/writes from/to DCC on more
+	  than one CPU core. Each core has its own DCC device registers, so when a
+	  CPU core reads or writes from/to DCC, it only accesses its own DCC device.
+	  Since kernel code can run on any CPU core, every time the kernel wants to
+	  write to the console, it might write to a different DCC.
+
+	  In SMP mode, external debuggers create multiple windows, and each window
+	  shows the DCC output only from that core's DCC. The result is that
+	  console output is either lost or scattered across windows.
+
+	  Enable this option only if you are sure that you do not need features like
+	  CPU hotplug to work. For example, during early chipset bringups without
+	  debug serial console support. If unsure, say N.
+
+config HVC_RISCV_SBI
+	bool "RISC-V SBI console support"
+	depends on RISCV_SBI && NONPORTABLE
+	select HVC_DRIVER
+	help
+	  This enables support for console output via RISC-V SBI calls, which
+	  is normally used only during boot to output printk.  This driver
+	  conflicts with real console drivers and should not be enabled on
+	  systems that directly access the console.
+
+	  If you don't know what do to here, say N.
+
+config HVCS
+	tristate "IBM Hypervisor Virtual Console Server support"
+	depends on PPC_PSERIES && HVC_CONSOLE
+	help
+	  Partitionable IBM Power5 ppc64 machines allow hosting of
+	  firmware virtual consoles from one Linux partition by
+	  another Linux partition.  This driver allows console data
+	  from Linux partitions to be accessed through TTY device
+	  interfaces in the device tree of a Linux partition running
+	  this driver.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called hvcs.  Additionally, this module
+	  will depend on arch specific APIs exported from hvcserver.ko
+	  which will also be compiled when this driver is built as a
+	  module.