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SubjectRe: [PATCH 2/6] x86: bug: prevent gcc distortions
On Fri, May 18, 2018 at 12:59 AM Peter Zijlstra <peterz@infradead.org>
wrote:

> This is an awesome hack, but is there really nothing we can do to make
> it more readable? Esp, that global asm doing the macro definition is a
> pain to read.

I actually find that macro to be *more* legible than what we do now,
although I'm not enamored with the pseudo-operation name ("__BUG_FLAGS").

That said, the C header code itself I don't love.

I wonder if we should just introduce a new assembler header file, and get
it included when processing compiler-generated asm. We already do that for
our _real_ *.S files, with a number of our header files having constants
and code for the asm case too, not just C.

But we could have an <asm/asm-macro.h> header file that has these kinds of
macros (or "pseudo-instructions") for assembly language cases, and then we
could just rely on them in inline asm.

Because if you want to see illegible, look at what we currently generate:

# kernel/exit.c:1761: BUG();
#APP
# 1761 "kernel/exit.c" 1
1: .byte 0x0f, 0x0b
.pushsection __bug_table,"aw"
2: .long 1b - 2b # bug_entry::bug_addr
.long .LC0 - 2b # bug_entry::file #
.word 1761 # bug_entry::line #
.word 0 # bug_entry::flags #
.org 2b+12 #
.popsection
# 0 "" 2
# 1761 "kernel/exit.c" 1
180: #
.pushsection .discard.unreachable
.long 180b - . #
.popsection

# 0 "" 2
#NO_APP

and tell me that's legible.. Of course, I'm probably one of the few people
who actually look at the generated asm fairly regularly.

So a few macros that we can use in inline asm definitely wouldn't hurt
legibility. And if we actually can put them in a header file as legible
code - instead of having to wrap them in a global "asm()" macro in C code,
they'd probably be legible at a source level too.

It's not just the bug_flags cases. It's things like jump labels too:

# ./arch/x86/include/asm/jump_label.h:36: asm_volatile_goto("1:"
#APP
# 36 "./arch/x86/include/asm/jump_label.h" 1
1:.byte 0x0f,0x1f,0x44,0x00,0
.pushsection __jump_table, "aw"
.balign 8
.quad 1b, .L71, __tracepoint_sched_process_free+8 + 0 #,,
.popsection

# 0 "" 2
#NO_APP

and atomics:

# ./arch/x86/include/asm/atomic.h:122: GEN_UNARY_RMWcc(LOCK_PREFIX
"decl", v->counter, "%0", e);
#APP
# 122 "./arch/x86/include/asm/atomic.h" 1
.pushsection .smp_locks,"a"
.balign 4
.long 671f - .
.popsection
671:
lock; decl -2336(%rbp) # _7->counter
/* output condition code e*/

# 0 "" 2
# ./include/linux/sched/task.h:95: if (atomic_dec_and_test(&t->usage))
#NO_APP

where I suspect we could hide the whole "lock" magic in a macro, and make
this much more legible.

Maybe? I think it might be worth trying. It's possible that the macro games
themselves would just cause enough pain to make any gains go away.

Linus

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