[PATCH 09/12] Eclair: deviate BUILD_ERROR() wrt rule 2.1 and introduce variants

BUILD_ERROR() is even stronger a guard than assertions in general, and
ASSERT_UNREACHABLE() (or BUG()) in particular. Deviate it just like those
to allow use for marking unreachable portions of code.

In some cases code being unreachable is dependent upon configuration.
Introduce two variants, as constructs like

    if ( IS_ENABLED(CONFIG_...) )
        BUILD_ERROR("...");

results in the if() still being reported as unreachable. Sadly these two
new macros introduce a new 20.12 violation each, which hence also needs
deviating.

Signed-off-by: Jan Beulich <[email protected]>

--- a/automation/eclair_analysis/ECLAIR/deviations.ecl
+++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
@@ -19,6 +19,7 @@ Constant expressions and unreachable bra
 
 -doc_begin="Unreachability inside an ASSERT_UNREACHABLE() and analogous macro calls is deliberate and safe."
 -config=MC3A2.R2.1,reports+={deliberate, "any_area(any_loc(any_exp(macro(name(ASSERT_UNREACHABLE||PARSE_ERR_RET||PARSE_ERR||FAIL_MSR||FAIL_CPUID)))))"}
+-config=MC3A2.R2.1,reports+={deliberate, "any_area(any_loc(any_exp(macro(^BUILD_ERROR(|_IF(|_NOT))$))))"}
 -doc_end
 
 -doc_begin="The asm-offset files are not linked deliberately, since they are used to generate definitions for asm modules."
@@ -667,6 +668,7 @@ deliberate."
 to the # or ## operators within the following macros are deliberate, to provide
 useful diagnostic messages to the user."
 -config=MC3A2.R20.12,macros+={deliberate, "name(ASSERT||BUILD_BUG_ON||BUILD_BUG_ON_ZERO||RUNTIME_CHECK)"}
+-config=MC3A2.R20.12,macros+={deliberate, "^BUILD_ERROR(|_IF(|_NOT))$"}
 -doc_end
 
 -doc_begin="The helper macro GENERATE_CASE may use a macro parameter for ordinary
--- a/xen/include/xen/macros.h
+++ b/xen/include/xen/macros.h
@@ -64,6 +64,21 @@
  */
 #define BUILD_ERROR(msg) asm ( ".error \"" msg "\"" )
 
+/*
+ * Like above, but conditional upon @cfg (not) being enabled.  @cfg must be
+ * suitable to pass to IS_ENABLED().
+ */
+#define BUILD_ERROR_IF(cfg)                               \
+    (IS_ENABLED(cfg)                                      \
+     ? ({ BUILD_ERROR( #cfg " unexpectedly enabled"); })  \
+     : (void)0)
+
+#define BUILD_ERROR_IF_NOT(cfg)                           \
+    (!IS_ENABLED(cfg)                                     \
+     ? ({ BUILD_ERROR( #cfg " unexpectedly disabled"); }) \
+     : (void)0)
+
+
 /* Hide a value from the optimiser. */
 #define HIDE(x)                                 \
     ({                                          \