VMX: use proper instruction mnemonics if assembler supports them

With the hex byte emission we were taking away a good part of
flexibility from the compiler, as for simplicity reasons these were
built using fixed operands. All half way modern build environments
would allow using the mnemonics (but we can't disable the hex variants
yet, since the binutils around at the time gcc 4.1 got released didn't
support these yet).

I didn't convert __vmread() yet because that would, just like for
__vmread_safe(), imply converting to a macro so that the output operand
can be the caller supplied variable rather than an intermediate one. As
that would require touching all invocation points of __vmread() (of
which there are quite a few), I'd first like to be certain the approach
is acceptable; the main question being whether the now conditional code
might be considered to cause future maintenance issues, and the second
being that of parameter/argument ordering (here I made __vmread_safe()
match __vmwrite(), but one could also take the position that read and
write should use the inverse order of one another, in line with the
actual instruction operands).

Additionally I was quite puzzled to find that all the asm()-s involved
here have memory clobbers - what are they needed for? Or can they be
dropped at least in some cases?

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/Config.mk b/Config.mk
index f169be482b..a26c7c0f2b 100644
--- a/Config.mk
+++ b/Config.mk
@@ -4,6 +4,12 @@ ifeq ($(filter /%,$(XEN_ROOT)),)
 $(error XEN_ROOT must be absolute)
 endif
 
+# Convenient variables
+comma   := ,
+squote  := '
+empty   :=
+space   := $(empty) $(empty)
+
 # fallback for older make
 realpath = $(wildcard $(foreach file,$(1),$(shell cd -P $(dir $(file)) && echo "$$PWD/$(notdir $(file))")))
 
@@ -128,6 +134,21 @@ endef
 check-$(gcc) = $(call cc-ver-check,CC,0x040100,"Xen requires at least gcc-4.1")
 $(eval $(check-y))
 
+# as-insn: Check whether assembler supports an instruction.
+# Usage: cflags-y += $(call as-insn "insn",option-yes,option-no)
+as-insn = $(if $(shell echo 'void _(void) { asm volatile ( $(2) ); }' \
+                       | $(1) -c -x c -o /dev/null - 2>&1),$(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)))
+define as-insn-check-closure
+    ifeq ($$(call as-insn,$$($(2)),$(3),y,n),y)
+        $(1) += $(4)
+    endif
+endef
+
 define buildmakevars2shellvars
     export PREFIX="$(PREFIX)";                                            \
     export XEN_SCRIPT_DIR="$(XEN_SCRIPT_DIR)";                            \