x86: reduce general stack alignment to 8

We don't need bigger alignment except when calling EFI boot or runtime
services functions (and we don't guarantee that either, as explained
close to the top of xen/common/efi/runtime.c in the struct efi_rs_state
declaration). Hence if the compiler supports reducing stack alignment
from the ABI compatible 16 bytes (gcc 7 and newer), do so wherever
possible.

The EFI case itself is largely dealt with already (actually forcing
32-byte alignment) as a result of commit f6b7fedc

 ("x86/EFI: meet
further spec requirements for runtime calls"). However, as explained in
the description of that earlier change, without using
-mincoming-stack-boundary=3 (which we don't want) we still have to make
the compiler assume 16-byte stack boundaries for CUs making EFI calls in
order to keep the compiler from aligning the stack, but then placing an
odd number of 8-byte objects on it, resulting in a mis-aligned outgoing
stack.

This as a side effect yields some code size reduction, since for a
number of sufficiently simple non-leaf functions the stack adjustment
(by 8, when there are no local stack variables at all) gets dropped
altogether. I notice exceptions though, for example in guest_cpuid(),
where in a release build gcc 8.2 now decides to set up a frame pointer
(without ever using %rbp); I consider this a compiler quirk which we
should leave to the compiler folks to address eventually.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

xen/arch/x86/Rules.mk

@@ -52,6 +52,11 @@ CFLAGS += -fno-jump-tables
 export CONFIG_INDIRECT_THUNK=y
 endif
 
+# If supported by the compiler, reduce stack alignment to 8 bytes. But allow
+# this to be overridden elsewhere.
+$(call cc-option-add,CFLAGS-stack-boundary,CC,-mpreferred-stack-boundary=3)
+CFLAGS += $(CFLAGS-stack-boundary)
+
 # Set up the assembler include path properly for older toolchains.
 CFLAGS += -Wa,-I$(BASEDIR)/include
 

xen/arch/x86/efi/Makefile

@@ -5,7 +5,11 @@ CFLAGS += -fshort-wchar
 
 boot.init.o: buildid.o
 
+EFIOBJ := boot.init.o compat.o runtime.o
+
+$(EFIOBJ): CFLAGS-stack-boundary := -mpreferred-stack-boundary=4
+
 obj-y := stub.o
-obj-$(XEN_BUILD_EFI) := boot.init.o compat.o relocs-dummy.o runtime.o
+obj-$(XEN_BUILD_EFI) := $(EFIOBJ) relocs-dummy.o
 extra-$(XEN_BUILD_EFI) += buildid.o
 nocov-$(XEN_BUILD_EFI) += stub.o