Re: API noodling

[Peter da Silva <peter-dragonfly@xxxxxxxxxxx>]

Matthew Dillon wrote:
> :Using a union won't give you prebuilt templates for the versionless
> :universal compatibility sceme you were talking about.

>     The union is really just a space allocation.  We can add some padding,
>     and anyway the emulation layer *will* be aware of the user program's
>     release since it's syscall entry points (e.g. 'read()') have to be
>     compatible.

Yeh, but I thought that the emulation would be providing the glue
between the application version and the kernel version by passing
a structured message to the kernel. I must have misunderstood
something somewhere in that message where you had something like
READ, ARG_HANDLE, fd, ARG_BUFPOINTER, address, ARG_SIZE, length...

> :>     A user thread will only be in one direct system call interface 
> :>     (i.e. read(), write(), gettimeofday()) at a time, so a single 
> :>     union'd syscall message structure embedded in the lwkt_thread is
> :>     sufficient.

> :Two... one for when it's in a signal... if signals aren't handled in a
> :separate thread.

>     The td_sysmsg idea was just for messages we might send, to optimize
>     standard system calls.  Incoming messages would be handled differently.
>     We don't have to use the pre-cache approach for all of our message types,
>     just for the ones in the critical path to streamline the interface.
>     Signals aren't really in the critical path so those could be allocated.

I'm looking at the case where a signal makes a system call: you don't 
want it re-using the buffer that the system call it interrupted used,
so the wrapper would need to do something like:

	if(in_a_signal_handler) {
		msgp = ...->td_sigmsg;
	} else {
		msgp = ...->td_sysmsg;
	}