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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/02/18 14:10, Roger Pau Monne wrote:
> 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@xxxxxxxxxx>
> ---
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Tim Deegan <tim@xxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
> 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)))

It is a shame about the naming.  This is really as-insn-add, even before
your changes.  I'll prepare a separate patch to rename it, which should
ideally come ahead of this patch.

>  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

What age is "Older" ?  As soon as these build fixes are in, I will
refresh my automatic padding calculation series, which will add skip
with labels to C code as well.

>  
>  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

This can end up adding -no-integrated-as twice, which would be more
obvious with as-insn-check renamed to as-insn-add.

I've got a plan to make this a lot cleaner.

~Andrew

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