x86: enable initial Rust support

Note that only x86_64 is covered and not all features nor mitigations
are handled, but it is enough as a starting point and showcases
the basics needed to add Rust support for a new architecture.

Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Co-developed-by: Alex Gaynor <[e-mail]>
Signed-off-by: Alex Gaynor <[e-mail]>
Co-developed-by: Wedson Almeida Filho <[e-mail]>
Signed-off-by: Wedson Almeida Filho <[e-mail]>
Co-developed-by: David Gow <[e-mail]>
Signed-off-by: David Gow <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>

Documentation/rust/arch-support.rst

···
 ============  ================  ==============================================
 Architecture  Level of support  Constraints
 ============  ================  ==============================================
+``x86``       Maintained        ``x86_64`` only.
 ============  ================  ==============================================

arch/x86/Kconfig

···
 	select HAVE_STATIC_CALL_INLINE		if HAVE_OBJTOOL
 	select HAVE_PREEMPT_DYNAMIC_CALL
 	select HAVE_RSEQ
+	select HAVE_RUST			if X86_64
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UACCESS_VALIDATION		if HAVE_OBJTOOL
 	select HAVE_UNSTABLE_SCHED_CLOCK

arch/x86/Makefile

···
 #    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
 #
 KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
+KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
 
 ifeq ($(CONFIG_X86_KERNEL_IBT),y)
 #
···
         cflags-$(CONFIG_GENERIC_CPU)	+= -mtune=generic
         KBUILD_CFLAGS += $(cflags-y)
 
+        rustflags-$(CONFIG_MK8)		+= -Ctarget-cpu=k8
+        rustflags-$(CONFIG_MPSC)	+= -Ctarget-cpu=nocona
+        rustflags-$(CONFIG_MCORE2)	+= -Ctarget-cpu=core2
+        rustflags-$(CONFIG_MATOM)	+= -Ctarget-cpu=atom
+        rustflags-$(CONFIG_GENERIC_CPU)	+= -Ztune-cpu=generic
+        KBUILD_RUSTFLAGS += $(rustflags-y)
+
         KBUILD_CFLAGS += -mno-red-zone
         KBUILD_CFLAGS += -mcmodel=kernel
+        KBUILD_RUSTFLAGS += -Cno-redzone=y
+        KBUILD_RUSTFLAGS += -Ccode-model=kernel
 endif
 
 #

scripts/generate_rust_target.rs

···
     let mut ts = TargetSpec::new();
 
     // `llvm-target`s are taken from `scripts/Makefile.clang`.
-    if cfg.has("DUMMY_ARCH") {
-        ts.push("arch", "dummy_arch");
+    if cfg.has("X86_64") {
+        ts.push("arch", "x86_64");
+        ts.push(
+            "data-layout",
+            "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
+        );
+        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
+        if cfg.has("RETPOLINE") {
+            features += ",+retpoline-external-thunk";
+        }
+        ts.push("features", features);
+        ts.push("llvm-target", "x86_64-linux-gnu");
+        ts.push("target-pointer-width", "64");
     } else {
         panic!("Unsupported architecture");
     }