Ingo Molnar wrote:
* H. Peter Anvin <[email protected]> wrote:
Why not just pin down the current ABI that there's 6 syscall
parameters _and not more_?
Because we have already violated it. There are system calls that need
more than 6 arguments: we need *a* convention. Worse, we're not
actually talking 6 *arguments*, we're talking 6 *words*; on 32-bit
platforms a single argument can occupy two words.
i think you are at least partly wrong here. Multiplexing/demultiplexing
can go on infinitely - for example sys_write(fd, size, buf) can be
thought of as a function call that passes in fd, size and a variable
number of arguments of the data to be written.
in that sense capping function arguments at 6 is _sensible_ because it
prefers _simple_ interfaces. When i wrote syslets i did a syscall number
of arguments histogram:
#args #syscalls
-----------------
0 22
1 51
2 83
3 85
4 40
5 23
6 8
Fortunately what we see today is that 80% of all syscalls have 4 or less
parameters. (yes, there are a few 6-parameter syscalls that arguably
hurt, but still, it's the exception not the rule)
this histogram shows a healthy bell curve which is _not_ limited by the
arguments limit of 6, but by common sense! If the 6-arguments limit was
a problem then we'd see a pile-up of 6-param syscalls.
so i believe you should start thinking about lots-of-arguments syscalls
as an exception not as something that needs to fit into some generic
ABI. (Especially as most schemes that were supposed to handle this
problem would hurt the sane 4-parameter (or less) syscall case too.)
I guess I'm confused here... all I said was I wanted them to be
systematic, and not need ad-hoc interfaces. In particular, I really
don't want to see an interface where "oh, the fifth parameter is really
a flags field so it's passed with sys_indirect, and is only accessible
via a sys_indirect" is the norm.
We don't have all that many; pselect() being the main one (I think there
might be a handful more on 32-bit platforms, but not positive.) It
introduced the convention of pointing argument register 6 to a
user-space data structure. Simple, and as you correctly point out, it's
a comparatively rare case. In klibc, I currently handle it as a special
case, but I would prefer to avoid special cases of that sort going forward.
Note that on s390, 6-parameter system calls are already a special case:
anything with over 5 parameters is invoked via a memory structure. This
actually means that for pselect on s390, we indirect via a memory
structure not once, but twice, for no good reason.
-hpa
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