[PATCH v5 3/5] x86/clang: restore integrated assembler usage with indirect thunks

[Roger Pau Monne <roger.pau@citrix.com>]

If the required features are met by the integrated clang assembler
(support for .includes and propagation of .macro-s between asm()-s)
do not disable it.

Only disable the integrated assembler for assembly files, like it was
done prior to "x86: Support indirect thunks from assembly code".

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Tim Deegan <tim@xen.org>
Cc: Wei Liu <wei.liu2@citrix.com>
---
Changes since v4:
 - Do not use else ifeq on the same line to be compatible with make
   3.8.
 - Modify as-insn-check to allow adding flags if the test case fails.

Changes since v3:
 - Do not modify how the thunk is included, clang upstream (and 6) has
   been fixed to propagate .macro-s between asm()-s.

Changes since v1:
 - Use as-insn to check if the assembler supports .include.
 - Open code a check for whether the assembler forgets .macro-s
   between asm()-s.
---
 Config.mk             |  9 +++++----
 xen/Rules.mk          |  5 +++--
 xen/arch/x86/Rules.mk | 14 ++++++++++++++
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/Config.mk b/Config.mk
index 407472c3fc..8d6d984488 100644
--- a/Config.mk
+++ b/Config.mk
@@ -162,13 +162,14 @@ as-insn = $(if $(shell echo 'void _(void) { asm volatile ( $(2) ); }' \
                        | $(filter-out -M% %.d -include %/include/xen/config.h,$(1)) \
                               -c -x c -o /dev/null - 2>&1),$(4),$(3))
 
-# as-insn-check: Add an option to compilation flags, but only if insn is
-#                supported by assembler.
-# Usage: $(call as-insn-check CFLAGS,CC,"nop",-DHAVE_GAS_NOP)
-as-insn-check = $(eval $(call as-insn-check-closure,$(1),$(2),$(3),$(4)))
+# as-insn-check: Conditionally add an option to compilation flags
+# Usage: $(call as-insn-check CFLAGS,CC,"nop",-DHAVE_GAS_NOP,-DNO_GAS_NOP)
+as-insn-check = $(eval $(call as-insn-check-closure,$(1),$(2),$(3),$(4),$(5)))
 define as-insn-check-closure
     ifeq ($$(call as-insn,$$($(2)) $$($(1)),$(3),y,n),y)
         $(1) += $(4)
+    else
+        $(1) += $(5)
     endif
 endef
 
diff --git a/xen/Rules.mk b/xen/Rules.mk
index 2918019b92..7adf757fb6 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -70,8 +70,9 @@ endif
 
 AFLAGS-y                += -D__ASSEMBLY__
 
-# Clang's built-in assembler can't handle embedded .include's
-CFLAGS-$(clang)         += -no-integrated-as
+# Older clang's built-in assembler doesn't understand .skip with labels:
+# https://bugs.llvm.org/show_bug.cgi?id=27369
+AFLAGS-$(clang)         += -no-integrated-as
 
 ALL_OBJS := $(ALL_OBJS-y)
 
diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index 1dc5c3785a..cee83d392e 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -44,3 +44,17 @@ endif
 
 # Set up the assembler include path properly for older toolchains.
 CFLAGS += -Wa,-I$(BASEDIR)/include
+
+ifeq ($(clang),y)
+    # Check whether clang asm()-s support .include.
+    $(call as-insn-check,CFLAGS,CC,".include \"asm/indirect_thunk_asm.h\"",, \
+                         -no-integrated-as)
+    # Check whether clang keeps .macro-s between asm()-s:
+    # https://bugs.llvm.org/show_bug.cgi?id=36110
+    ifeq ($(if $(shell echo 'void _(void) { asm volatile ( ".macro FOO\n.endm" ); \
+                                            asm volatile ( ".macro FOO\n.endm" ); }' \
+                       | $(CC) $(filter-out -M% %.d -include %/include/xen/config.h,$(CFLAGS)) \
+                               -c -x c -o /dev/null - 2>&1),n,y),y)
+        CFLAGS += -no-integrated-as
+    endif
+endif
-- 
2.16.1


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