On Wed, Feb 14, 2007 at 11:50:39AM +0100, Ingo Molnar wrote:
> * 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.
Let me spell it out, since you appear to have completely missed my point.
At the moment, SKIP_TO_NEXT_ON_STOP is specified to jump a "jump a full
syslet_uatom number of bytes".
If we end up with a system call being added which requires more than
the currently allowed number of arguments (and it _has_ happened before)
then either those syscalls are not accessible to syslets, or you need
to increase the arg_ptr array.
That makes syslet_uatom larger.
If syslet_uatom is larger, SKIP_TO_NEXT_ON_STOP increments the syslet_uatom
pointer by a greater number of bytes.
If we're running a set of userspace syslets built for an older kernel on
such a newer kernel, that is an incompatible change which will break.
> > 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[].
Okay - I guess the userspace library approach is fine, but it needs
to be documented that applications which build syslets directly are
going to be non-portable.
--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of:
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