Re: [Xen-devel] [PATCH v2 1/3] livepach: Add .livepatch.hooks functions and test-case

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.08.16 at 20:01, <konrad.wilk@xxxxxxxxxx> wrote:
>> >> > @@ -70,7 +71,11 @@ struct payload {
>> >> >      unsigned int nsyms;                  /* Nr of entries in .strtab 
>> >> > and 
>> >> > symbols. */
>> >> >      struct livepatch_build_id id;        /* 
>> >> > ELFNOTE_DESC(.note.gnu.build-id) of the payload. */
>> >> >      struct livepatch_build_id dep;       /* 
>> >> > ELFNOTE_DESC(.livepatch.depends). */
>> >> > -    char name[XEN_LIVEPATCH_NAME_SIZE]; /* Name of it. */
>> >> > +    livepatch_loadcall_t **load_funcs;   /* The array of funcs to call 
>> >> > after */
>> >> > +    livepatch_unloadcall_t **unload_funcs;/* load and unload of the 
>> >> > payload. */
>> >> 
>> >> These both seem like they want a const in the middle.
>> > 
>> > I did that initially and found out that the calling ->load_funcs[i] 
>> > resulted
>> > in _no_ code being called.
>> > 
>> > I did not dig in the assembler output to figure out what happend, let me
>> > do that now.
>> 
>> Indeed - I can't see how a const modifier here could alter whether
>> or not the pointed to function gets called.
> 
> I did a diff on the pre-assemble part (-S) with and without const on the above
> structs. With 'const' we get:
> 
>         call    spin_debug_disable
>  .LVL163:
> -       .loc 1 1093 0
> -       cmpl    $0, 324(%rbx)
> -       je      .L112
> -       movl    $0, %r12d
> -.LVL164:
> -.L113:
> -       .loc 1 1094 0
> -       mov     %r12d, %edx
> -       movq    312(%rbx), %rax
> -       call    *(%rax,%rdx,8)
> -       .loc 1 1093 0
> -       addl    $1, %r12d
> -.LVL165:
> -       cmpl    %r12d, 324(%rbx)
> -       ja      .L113
> -.LVL166:
> -.L112:
> 
> being removed.
> 
> I thought it may have to do with the function not returning anything. But
> even after making the typedef return it still omitted the call 
> data->load_funcs[i]();

Odd. I've tried this simple example:

typedef int fn_t(void);

struct s {
        unsigned n;
        fn_t**fn;
        fn_t*const*fnc;
        const fn_t**cfn;
};

int test1(const struct s*ps) {
        unsigned i;
        int rc = 0;

        for(i = 0; !rc && i < ps->n; ++i)
                rc = ps->fn[i]();

        return rc;
}

int test2(const struct s*ps) {
        unsigned i;
        int rc = 0;

        for(i = 0; !rc && i < ps->n; ++i)
                rc = ps->fnc[i]();

        return rc;
}

int test3(const struct s*ps) {
        unsigned i;
        int rc = 0;

        for(i = 0; !rc && i < ps->n; ++i)
                rc = ps->cfn[i]();

        return rc;
}

test1() and test2() get compiled identically. test3(), using the field
with the misplaced const, oddly enough gets compiled slightly
differently (and without a warning despite one would seem
warranted), yet the call doesn't get omitted. If, however, I change
the return type of fn_t to void, the function body of test3() ends
up empty, which is a compiler bug afaict, but which also suggests
that you've tried the variant with the misplaced const.

Jan


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