* Russell King <[email protected]> wrote:
> On Tue, Feb 13, 2007 at 03:20:42PM +0100, Ingo Molnar wrote:
> > +Arguments to the system call are implemented via pointers to arguments.
> > +This not only increases the flexibility of syslet atoms (multiple syslets
> > +can share the same variable for example), but is also an optimization:
> > +copy_uatom() will only fetch syscall parameters up until the point it
> > +meets the first NULL pointer. 50% of all syscalls have 2 or less
> > +parameters (and 90% of all syscalls have 4 or less parameters).
> > +
> > + [ Note: since the argument array is at the end of the atom, and the
> > + kernel will not touch any argument beyond the final NULL one, atoms
> > + might be packed more tightly. (the only special case exception to
> > + this rule would be SKIP_TO_NEXT_ON_STOP atoms, where the kernel will
> > + jump a full syslet_uatom number of bytes.) ]
>
> What if you need to increase the number of arguments passed to a
> system call later? That would be an API change since the size of
> syslet_uatom would change?
the syslet_uatom has a constant size right now, and space for a maximum
of 6 arguments. /If/ the user knows that a specific atom (which for
example does a sys_close()) takes only 1 argument, it could shrink the
size of the atom down by 4 arguments.
[ i'd not actually recommend doing this, because it's generally a
volatile thing to play such tricks - i guess i shouldnt have written
that side-note in the header file :-) ]
there should be no new ABI issues: the existing syscall ABI never
changes, it's only extended. New syslets can rely on new properties of
new system calls. This is quite parallel to how glibc handles system
calls.
> How do you propose syslet users know about these kinds of ABI issues
> (including the endian-ness of 64-bit arguments) ?
syslet users would preferably be libraries like glibc - not applications
- i'm not sure the raw syslet interface should be exposed to
applications. Thus my current thinking is that syslets ought to be
per-arch structures - no need to pad them out to 64 bits on 32-bit
architectures - it's per-arch userspace that makes use of them anyway.
system call encodings are fundamentally per-arch anyway - every arch
does various fixups and has its own order of system calls.
but ... i'd not be against having a 'generic syscall layer' though, and
syslets might be a good starting point for that. But that would
necessiate a per-arch table of translating syscall numbers into this
'generic' numbering, at minimum - or a separate sys_async_call_table[].
Ingo
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