Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

[Alan Cox <gnomes@lxorguk.ukuu.org.uk>]

> and the *implementation* might be architecture-specific, but one
> particular implementation would be something like simply
> 
> #define array_access(base, idx, max) ({                         \
>         union { typeof(base[0]) _val; unsigned long _bit; } __u;\
>         unsigned long _i = (idx);                               \
>         unsigned long _m = (max);                               \
>         unsigned long _mask = _i < _m ? ~0 : 0;                 \
>         OPTIMIZER_HIDE_VAR(_mask);                              \
>         __u._val = base[_i & _mask];                            \
>         __u._bit &= _mask;                                      \
>         __u._val; })
> 
> (Ok, the above is not exhaustively tested, but you get the idea, and
> gcc doesn't seem to mess it up *too* badly).

How do you ensure that the CPU doesn't speculate j < _m  ? ~0 : 0 pick the
wrong mask and then reference base[] ?

It's a nice idea but I think we'd need to run it past the various CPU
vendors especially before it was implemented as a generic solution.
Anding with a constant works because the constant doesn't get speculated
and nor does the and with a constant, but you've got a whole additional
conditional path in your macro.

Alan