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: - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
