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SubjectRe: hammer: MAP_32BIT


Ulrich Drepper wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Timothy Miller wrote:
>
>
>>If your program is capable of handling an address with more than 32
>>bits, what point is there giving a hint? Either your program can handle
>>64-bit pointers or it cannot. Any program flexible enough to handle
>>either size dynamically would expend enough overhead checking that it
>>would be worse than if it just made a hard choice.
>
>
> Look at the x86-64 context switching code. If memory addressed by the
> GDT entries has a 32-bit address it uses a different method than for
> cases where the virtual address has more than 32 bits. This way of
> handling GDT entries is faster according to ak. So, it's not a
> correctness thing, it's a performance thing.
>

Alright. Sounds great. So my next question is this:

Why does there ever need to be an explicit HINT that you would prefer a
<32 bit address, when it's known a priori that <32 is better? Why
doesn't the mapping code ALWAYS try to use 32-bit addresses before
resorting to 64-bit?

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