On Mon, Apr 24, 2017 at 06:02:51PM +0200, Jiri Slaby wrote:
> On 04/24/2017, 05:55 PM, Ingo Molnar wrote:
> > * Jiri Slaby <jsl...@suse.cz> wrote:
> >
> >> On 04/24/2017, 05:08 PM, David Miller wrote:
> >>> If you align the entry points, then the code sequence as a whole is
> >>> are no longer densely packed.
> >>
> >> Sure.
> >>
> >>> Or do I misunderstand how your macros work?
> >>
> >> Perhaps. So the suggested macros for the code are:
> >> #define BPF_FUNC_START_LOCAL(name) \
> >> SYM_START(name, SYM_V_LOCAL, SYM_A_NONE)
> >> #define BPF_FUNC_START(name) \
> >> SYM_START(name, SYM_V_GLOBAL, SYM_A_NONE)
> >>
> >> and they differ from the standard ones:
> >> #define SYM_FUNC_START_LOCAL(name) \
> >> SYM_START(name, SYM_V_LOCAL, SYM_A_ALIGN)
> >> #define SYM_FUNC_START(name) \
> >> SYM_START(name, SYM_V_GLOBAL, SYM_A_ALIGN)
> >>
> >>
> >> The difference is SYM_A_NONE vs. SYM_A_ALIGN, which means:
> >> #define SYM_A_ALIGN ALIGN
> >> #define SYM_A_NONE /* nothing */
> >>
> >> Does it look OK now?
> >
> > No, the patch changes alignment which is undesirable, it needs to preserve
> > the
> > existing (non-)alignment of the symbols!
>
> OK, so I am not expressing myself explicitly enough, it seems.
>
> So, correct, the patch v3 adds alignments. I suggested in the discussion
> the macros above. They do not add alignments. If everybody is OK with
> that, v4 of the patch won't add alignments. OK?
can we go back to what problem this patch set is trying to solve?
Sounds like you want to add _function_ start/end marks to aid debugging?
Debugging with what? What tool will recognize this stuff?
Take a look at what your patch does:
+ENTRY(sk_load_word)
test %esi,%esi
js bpf_slow_path_word_neg
+ENDPROC(sk_load_word)
Does above two assembler instructions look like a function?
or this:
+ENTRY(sk_load_byte_positive_offset)
cmp %esi,%r9d /* if (offset >= hlen) goto bpf_slow_path_byte */
jle bpf_slow_path_byte
movzbl (SKBDATA,%rsi),%eax
ret
+ENDPROC(sk_load_byte_positive_offset)
This assembler code doesn't represent functions. There is no prologue/epilogue
and no stack frame. JITed code uses 'call' insn to jump into them, but they're
not your typical C functions.
Take a look at bpf_slow_path_common() macro that creates the frame before
calling into C code with 'call skb_copy_bits;'
I still think that this code should be left alone.
Even macro names you're proposing:
#define BPF_FUNC_START_LOCAL
don't sound right. These are not functions.
