[PATCH v1] tools build: Fix rust cross compilation

[Dmitrii Dolgov <9erthalion6@gmail.com>]

Currently no target is specified to compile rust code when needed, which
breaks cross compilation. Detect required target and pass it via
rust_flags to the compiler.

Note that CROSS_COMPILE might be different from what rust compiler
expects, since it may omit the target vendor value, e.g.
"aarch64-linux-gnu" instead of "aarch64-unknown-linux-gnu". Thus
explicitly map supported CROSS_COMPILE values to corresponding Rust
versions, as suggested by Miguel Ojeda.

Tested using arm64 cross-compilation example from [1].

[1]: https://perfwiki.github.io/main/arm64-cross-compilation-dockerfile/

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
---
 tools/build/Build.include  |  9 +++++++++
 tools/perf/Makefile.config | 14 ++++++++++++++
 tools/perf/Makefile.perf   |  2 +-
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/tools/build/Build.include b/tools/build/Build.include
index e45b2eb0d24..cd0baa7a168 100644
--- a/tools/build/Build.include
+++ b/tools/build/Build.include
@@ -98,6 +98,15 @@ c_flags_2 = $(filter-out $(CFLAGS_REMOVE_$(basetarget).o), $(c_flags_1))
 c_flags   = $(filter-out $(CFLAGS_REMOVE_$(obj)), $(c_flags_2))
 cxx_flags = -Wp,-MD,$(depfile) -Wp,-MT,$@ $(CXXFLAGS) -D"BUILD_STR(s)=\#s" $(CXXFLAGS_$(basetarget).o) $(CXXFLAGS_$(obj))
 
+###
+# Rust flags to be used on rule definition, includes:
+# - global $(RUST_FLAGS)
+# - per target Rust flags
+# - per object Rust flags
+rust_flags_1 = $(RUST_FLAGS) $(RUST_FLAGS_$(basetarget).o) $(RUST_FLAGS_$(obj))
+rust_flags_2 = $(filter-out $(RUST_FLAGS_REMOVE_$(basetarget).o), $(rust_flags_1))
+rust_flags   = $(filter-out $(RUST_FLAGS_REMOVE_$(obj)), $(rust_flags_2))
+
 ###
 ## HOSTCC C flags
 
diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index a8dc72cfe48..f40c5a70520 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -1163,6 +1163,20 @@ ifndef NO_RUST
     CFLAGS += -DHAVE_RUST_SUPPORT
     $(call detected,CONFIG_RUST_SUPPORT)
   endif
+
+  ifneq ($(CROSS_COMPILE),)
+    RUST_TARGET_FLAGS_arm	:= arm-unknown-linux-gnueabi
+    RUST_TARGET_FLAGS_arm64	:= aarch64-unknown-linux-gnu
+    RUST_TARGET_FLAGS_m68k	:= m68k-unknown-linux-gnu
+    RUST_TARGET_FLAGS_mips	:= mipsel-unknown-linux-gnu
+    RUST_TARGET_FLAGS_powerpc	:= powerpc64le-unknown-linux-gnu
+    RUST_TARGET_FLAGS_riscv	:= riscv64-unknown-linux-gnu
+    RUST_TARGET_FLAGS_s390	:= s390x-unknown-linux-gnu
+    RUST_TARGET_FLAGS_x86	:= x86_64-unknown-linux-gnu
+    RUST_TARGET_FLAGS_x86_64	:= x86_64-unknown-linux-gnu
+
+    RUST_FLAGS += --target=$(RUST_TARGET_FLAGS_$(ARCH))
+  endif
 endif
 
 # Among the variables below, these:
diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index a6d8ca3e923..f3919838243 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -271,7 +271,7 @@ ifeq ($(PYLINT),1)
   PYLINT := $(shell which pylint 2> /dev/null)
 endif
 
-export srctree OUTPUT RM CC CXX RUSTC LD AR CFLAGS CXXFLAGS V BISON FLEX AWK
+export srctree OUTPUT RM CC CXX RUSTC LD AR CFLAGS CXXFLAGS RUST_FLAGS V BISON FLEX AWK
 export HOSTCC HOSTLD HOSTAR HOSTCFLAGS SHELLCHECK MYPY PYLINT
 
 include $(srctree)/tools/build/Makefile.include

base-commit: 5d1ab659fb93eed85d6d8b2937013360157032f4
-- 
2.52.0