Messages in this thread | | | Date | Wed, 14 Feb 2007 11:50:39 +0100 | From | Ingo Molnar <> | Subject | Re: [patch 06/11] syslets: core, documentation |
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* Russell King <rmk+lkml@arm.linux.org.uk> 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[].
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