[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 provide
an explicit RUST_TARGET_ARCH, and if not specified, try to match
the CROSS_COMPILE value.

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 | 18 ++++++++++++++++++
 tools/perf/Makefile.perf   |  4 +++-
 3 files changed, 30 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..9b164b442da 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -1163,6 +1163,24 @@ ifndef NO_RUST
     CFLAGS += -DHAVE_RUST_SUPPORT
     $(call detected,CONFIG_RUST_SUPPORT)
   endif
+
+  ifneq ($(CROSS_COMPILE),)
+    ifneq ($(RUST_TARGET_ARCH),)
+      RUST_FLAGS += --target=$(RUST_TARGET_ARCH)
+    else
+      # CROSS_COMPILE specifies the target triple, which may not match what
+      # Rust expects, e.g. it can omit the target vendor if C toolchain is
+      # used. For example the value can be aarch64-linux-gnu, where Rust uses
+      # aarch64-unknown-linux-gnu. Try to match that and insert an "unknown"
+      # vendor if omitted.
+      TRIPLE := $(subst -, ,$(CROSS_COMPILE:%-=%))
+      ifeq ($(words $(TRIPLE)),3)
+        RUST_FLAGS += --target=$(word 1,$(TRIPLE))-unknown-$(word 2,$(TRIPLE))-$(word 3,$(TRIPLE))
+      else
+        RUST_FLAGS += --target=$(CROSS_COMPILE:%-=%)
+      endif
+    endif
+  endif
 endif
 
 # Among the variables below, these:
diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index a6d8ca3e923..621800e1716 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -17,6 +17,8 @@ include ../scripts/utilities.mak
 #
 # Define CROSS_COMPILE as prefix name of compiler if you want cross-builds.
 #
+# Define RUST_TARGET_ARCH as rust target arch if you want cross-builds.
+#
 # Define LIBPERL to enable perl script extension.
 #
 # Define NO_LIBPYTHON to disable python script extension.
@@ -271,7 +273,7 @@ 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