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

[Steven Rostedt <rostedt@goodmis.org>]

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