Re: Intel P6 vs P7 system call performance

[Hugh Dickins]

On Mon, 16 Dec 2002, Linus Torvalds wrote:
> 
> Ok, I did the vsyscall page too, and tried to make it do the right thing
> (but I didn't bother to test it on a non-SEP machine).
> 
> I'm pushing the changes out right now, but basically it boils down to the
> fact that with these changes, user space can instead of doing an
> 
> 	int $0x80
> 
> instruction for a system call just do a
> 
> 	call 0xfffff000

I thought that last page was intentionally left invalid?

So that, for example, *(char *)MAP_FAILED will give SIGSEGV;
whereas now I can read a 0 there (and perhaps you should be
using get_zeroed_page rather than __get_free_page?).

I cannot name anything which relies on that page being invalid,
but think it would be safer to keep that it way; though I guess
more compatibility pain to use the next page down (or could
seg lim be used? I forget the granularity restrictions).

Hugh