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by monocasa
702 days ago
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Not quite. There isn't a door_recv(2) systemcall or equivalent. Doors truly don't transfer messages, they transfer the thread itself. As in the thread that made a door call is now just directly executing in the address space of the callee. They're more like i432/286/mill cpu task gates. |
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In somewhat anachronistic verbiage (at least in a modern software context) this may be true, but today this statement makes it sounds like code from the caller process is executing in the address space of the callee process, such that miraculously the caller code now can directly reference data in the callee. AFAICT that just isn't the case, and wouldn't even make sense--i.e. how would it know the addresses without a ton of complex reflection that's completely absent from example code? (Caller and callee don't need to have been forked from each other.) And according to the Linux implementation, the "argument" (a flat, contiguous block of data) passed from caller to callee is literally copied, either directly or by mapping in the pages. The caller even needs to provide a return buffer for the callee's returned data to be copied into (unless it's too large, then it's mapped in and the return argument vector updated to point to the newly mmap'd pages). File descriptors can also be passed, and of course that requires kernel involvement.
AFAICT, the trick here pertains to scheduling alone, both wrt to the hardware and software systems. I.e. a lighter weight interface for the hardware task gating mechanism, like you say, reliant on the synchronous semantics of this design to skip involving the system scheduler. But all the other process attributes, including address space, are switched out, perhaps in an optimized matter as mentioned elsethread but still preserving typical process isolation semantics.
If I'm wrong, please correct me with pointers to more detailed technical documentation (Or code--is this still in Illuminos?) because I'd love to dig more into it.
FWIW, Here's the Solaris man page for libdoor: https://docs.oracle.com/cd/E36784_01/html/E36873/libdoor-3li... Did you mean door_call or door_return instead of door_recv?