Re: [RFC] gcc feature request: Moving blocks into sections

Steven Rostedt
2013-08-05T21:44:16Z

On Mon, 2013-08-05 at 17:28 -0400, Mathieu Desnoyers wrote:

> Another thing that bothers me with Steven's approach is that decoding
> jumps generated by the compiler seems fragile IMHO.

The encodings wont change. If they do, then old kernels will not run on
new hardware.

Now if it adds a third option to jmp, then we hit the "die" path and
know right away that it wont work anymore. Then we fix it properly.

>
> x86 decoding proposed by https://lkml.org/lkml/2012/3/8/464 :
>
> +static int make_nop_x86(void *map, size_t const offset)
> +{
> + unsigned char *op;
> + unsigned char *nop;
> + int size;
> +
> + /* Determine which type of jmp this is 2 byte or 5. */
> + op = map + offset;
> + switch (*op) {
> + case 0xeb: /* 2 byte */
> + size = 2;
> + nop = ideal_nop2_x86;
> + break;
> + case 0xe9: /* 5 byte */
> + size = 5;
> + nop = ideal_nop;
> + break;
> + default:
> + die(NULL, "Bad jump label section (bad op %x)\n", *op);
> + __builtin_unreachable();
> + }
>
> My though is that the code above does not cover all jump encodings that
> can be generated by past, current and future x86 assemblers.
>
> Another way around this issue might be to keep the instruction size
> within a non-allocated section:
>
> static __always_inline bool arch_static_branch(struct static_key *key)
> {
> asm goto("1:"
> "jmp %l[l_yes]\n\t"
> "2:"
>
> ".pushsection __jump_table, \"aw\" \n\t"
> _ASM_ALIGN "\n\t"
> _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
> ".popsection \n\t"
>
> ".pushsection __jump_table_ilen \n\t"
> _ASM_PTR "1b \n\t" /* Address of the jmp */
> ".byte 2b - 1b \n\t" /* Size of the jmp instruction */
> ".popsection \n\t"
>
> : : "i" (key) : : l_yes);
> return false;
> l_yes:
> return true;
> }
>
> And use (2b - 1b) to know what size of no-op should be used rather than
> to rely on instruction decoding.
>
> Thoughts ?
>

Then we need to add yet another table of information to the kernel that
needs to hang around. This goes with another kernel-discuss request
talking about kernel data bloat.

-- Steve