[PATCH v3] XEN: enable MC/DC coverage for Clang

[Saman Dehghan <samaan.dehghan@xxxxxxxxx>]

Clang >= 18 supports Modified Condition/Decision Coverage (MC/DC).
This patch enables the detection and usage of this feature when
compiling Xen with Clang.

- Update detection logic in Kconfig to check for the required set of
  Clang flags for MC/DC:
  '-fprofile-instr-generate -fcoverage-mapping -fcoverage-mcdc'.
  This bundle is necessary because '-fcoverage-mcdc' requires
  '-fcoverage-mapping', which in turn requires '-fprofile-instr-generate'.
- Update llvm.c to handle the profile format changes (bitmap section)
  required for MC/DC.
- Guard -Wno-error=coverage-too-many-conditions with CONFIG_CC_IS_GCC
  to avoid passing a GCC-only warning option to Clang

Signed-off-by: Saman Dehghan <samaan.dehghan@xxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/Kconfig                |  9 +++++++--
 xen/Rules.mk               |  1 +
 xen/arch/x86/Makefile      |  2 +-
 xen/common/coverage/llvm.c | 18 +++++++++++++++++-
 4 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/xen/Kconfig b/xen/Kconfig
index a5e5af3b76..8f2cc111cd 100644
--- a/xen/Kconfig
+++ b/xen/Kconfig
@@ -51,9 +51,14 @@ config CC_HAS_ASM_GOTO_OUTPUT
        depends on !GCC_ASM_GOTO_OUTPUT_BROKEN
        depends on $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: 
bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
 
-# Compiler supports -fcondition-coverage aka MC/DC
+# Compiler supports Modified Condition/Decision Coverage (MC/DC).
+# MC/DC is a rigorous code coverage metric that requires every condition
+# within a decision (boolean expression) to be shown to independently
+# influence the decision's final outcome.
+#
+# Minimum toolchain baseline: GCC >= 14, or Clang >= 18.
 config CC_HAS_MCDC
-       def_bool $(cc-option,-fcondition-coverage)
+       def_bool $(cc-option,-fcondition-coverage) || 
$(cc-option,-fprofile-instr-generate -fcoverage-mapping -fcoverage-mcdc)
 
 # Set code alignment.
 #
diff --git a/xen/Rules.mk b/xen/Rules.mk
index 24f447b957..2b28d1ac3c 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -136,6 +136,7 @@ non-init-objects = $(filter-out %.init.o, $(obj-y) 
$(obj-bin-y) $(extra-y))
 
 ifeq ($(CONFIG_CC_IS_CLANG),y)
     cov-cflags-$(CONFIG_COVERAGE) := -fprofile-instr-generate 
-fcoverage-mapping
+    cov-cflags-$(CONFIG_CONDITION_COVERAGE) += -fcoverage-mcdc
 else
     cov-cflags-$(CONFIG_COVERAGE) := -fprofile-arcs -ftest-coverage
     cov-cflags-$(CONFIG_CONDITION_COVERAGE) += -fcondition-coverage
diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index 407571c510..6c0ff67fa8 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -98,7 +98,7 @@ $(obj)/usercopy.o: CFLAGS-y += -iquote .
 ifneq ($(CONFIG_HVM),y)
 $(obj)/x86_emulate.o: CFLAGS-y += -Wno-unused-label
 endif
-ifeq ($(CONFIG_CONDITION_COVERAGE),y)
+ifeq ($(CONFIG_CONDITION_COVERAGE)$(CONFIG_CC_IS_GCC),yy)
 $(obj)/x86_emulate.o: CFLAGS-y += -Wno-error=coverage-too-many-conditions
 endif
 
diff --git a/xen/common/coverage/llvm.c b/xen/common/coverage/llvm.c
index 532889c857..5663fb10dd 100644
--- a/xen/common/coverage/llvm.c
+++ b/xen/common/coverage/llvm.c
@@ -120,6 +120,8 @@ extern const char __start___llvm_prf_names[];
 extern const char __stop___llvm_prf_names[];
 extern uint64_t __start___llvm_prf_cnts[];
 extern uint64_t __stop___llvm_prf_cnts[];
+extern const char __start___llvm_prf_bits[];
+extern const char __stop___llvm_prf_bits[];
 
 #define START_DATA      ((const void *)__start___llvm_prf_data)
 #define END_DATA        ((const void *)__stop___llvm_prf_data)
@@ -127,16 +129,23 @@ extern uint64_t __stop___llvm_prf_cnts[];
 #define END_NAMES       ((const void *)__stop___llvm_prf_names)
 #define START_COUNTERS  ((void *)__start___llvm_prf_cnts)
 #define END_COUNTERS    ((void *)__stop___llvm_prf_cnts)
+#define START_BITMAP    ((void *)__start___llvm_prf_bits)
+#define END_BITMAP      ((void *)__stop___llvm_prf_bits)
 
 static void cf_check reset_counters(void)
 {
     memset(START_COUNTERS, 0, END_COUNTERS - START_COUNTERS);
+    if ( IS_ENABLED(CONFIG_CONDITION_COVERAGE) )
+        memset(START_BITMAP, 0, END_BITMAP - START_BITMAP);
 }
 
 static uint32_t cf_check get_size(void)
 {
-    return ROUNDUP(sizeof(struct llvm_profile_header) + END_DATA - START_DATA +
+    uint32_t size = ROUNDUP(sizeof(struct llvm_profile_header) + END_DATA - 
START_DATA +
                    END_COUNTERS - START_COUNTERS + END_NAMES - START_NAMES, 8);
+    if ( IS_ENABLED(CONFIG_CONDITION_COVERAGE) )
+        size += ROUNDUP(END_BITMAP - START_BITMAP, 8);
+    return size;
 }
 
 static int cf_check dump(
@@ -155,6 +164,10 @@ static int cf_check dump(
 #endif
         .names_delta = (uintptr_t)START_NAMES,
         .value_kind_last = LLVM_PROFILE_NUM_KINDS - 1,
+#if defined(CONFIG_CONDITION_COVERAGE) && LLVM_PROFILE_VERSION >= 9
+        .num_bitmap_bytes = END_BITMAP - START_BITMAP,
+        .bitmap_delta = START_BITMAP - START_DATA,
+#endif
     };
     unsigned int off = 0;
 
@@ -168,6 +181,9 @@ static int cf_check dump(
     APPEND_TO_BUFFER(&header, sizeof(header));
     APPEND_TO_BUFFER(START_DATA, END_DATA - START_DATA);
     APPEND_TO_BUFFER(START_COUNTERS, END_COUNTERS - START_COUNTERS);
+#if defined(CONFIG_CONDITION_COVERAGE)
+    APPEND_TO_BUFFER(START_BITMAP, END_BITMAP - START_BITMAP);
+#endif
     APPEND_TO_BUFFER(START_NAMES, END_NAMES - START_NAMES);
 #undef APPEND_TO_BUFFER
 
-- 
2.49.0