On Thu, 26 May 2011, Dave Martin wrote:
> > The core logic spans 5 instructions. Only 3 of them are actually the
> > same in both cases i.e. those with no references to 1b or 2b, and
> > they're perfectly interlaced in between the others. Trying to make this
> > into common runtime code would result in even bigger code I'm afraid.
> > This could be made into common source code via a macro though.
>
> Fair enough -- a macro might be worth a try _if_ it simplifies things
> in the source, but I think you're right that merging the actual code
> probably isn't worth it just to save a few words in the vectors page
> (which eats up 4K regardless of what we put in it) and a few cycles per
> fault (which already costs many, many cycles).
In the normal cases, there is no additional cycles per fault as the
inline check remains unchanged, and it goes like this:
@ Make sure our user space atomic helper is restarted
@ if it was interrupted in a critical region. Here we
@ perform a quick test inline since it should be false
@ 99.9999% of the time. The rest is done out of line.
cmp r2, #TASK_SIZE
blhs kuser_cmpxchg_fixup
In most cases the branch is not taken.
Nicolas
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