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, 58 insertions, 0 deletions

diff --git a/include/kvm/iodev.h b/include/kvm/iodev.h
new file mode 100644
index 000000000..56619e332
--- /dev/null
+++ b/include/kvm/iodev.h
@@ -0,0 +1,58 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef __KVM_IODEV_H__
+#define __KVM_IODEV_H__
+
+#include <linux/kvm_types.h>
+#include <linux/errno.h>
+
+struct kvm_io_device;
+struct kvm_vcpu;
+
+/**
+ * kvm_io_device_ops are called under kvm slots_lock.
+ * read and write handlers return 0 if the transaction has been handled,
+ * or non-zero to have it passed to the next device.
+ **/
+struct kvm_io_device_ops {
+	int (*read)(struct kvm_vcpu *vcpu,
+		    struct kvm_io_device *this,
+		    gpa_t addr,
+		    int len,
+		    void *val);
+	int (*write)(struct kvm_vcpu *vcpu,
+		     struct kvm_io_device *this,
+		     gpa_t addr,
+		     int len,
+		     const void *val);
+	void (*destructor)(struct kvm_io_device *this);
+};
+
+
+struct kvm_io_device {
+	const struct kvm_io_device_ops *ops;
+};
+
+static inline void kvm_iodevice_init(struct kvm_io_device *dev,
+				     const struct kvm_io_device_ops *ops)
+{
+	dev->ops = ops;
+}
+
+static inline int kvm_iodevice_read(struct kvm_vcpu *vcpu,
+				    struct kvm_io_device *dev, gpa_t addr,
+				    int l, void *v)
+{
+	return dev->ops->read ? dev->ops->read(vcpu, dev, addr, l, v)
+				: -EOPNOTSUPP;
+}
+
+static inline int kvm_iodevice_write(struct kvm_vcpu *vcpu,
+				     struct kvm_io_device *dev, gpa_t addr,
+				     int l, const void *v)
+{
+	return dev->ops->write ? dev->ops->write(vcpu, dev, addr, l, v)
+				 : -EOPNOTSUPP;
+}
+
+#endif /* __KVM_IODEV_H__ */