On the design of the SYSENTER Intel syscall instruction, and how the elegant idea fell apart https://groups.google.com/forum/m/#!topic/comp.arch/CjDs4MJCBow%5B1-25%5D …pic.twitter.com/VIZlsh87cE
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@SamuelAFalvoII @FioraAeterna If the syscall was already entitled to clobber those regs, signal handler is entitled to just save junk.
@RichFelker @FioraAeterna Fair enough; another question arises though: context switches. Many OSes use syscall returns to switch contexts.
@SamuelAFalvoII @FioraAeterna Again, if you switch contexts at syscall time & syscall clobbers regs, those regs don't need to be saved.
@SamuelAFalvoII @FioraAeterna Registers only need to be saved when they're meaningful to the (asynchronously) interrupted task.
@RichFelker @FioraAeterna There's no way to know what registers are needed by the (asynchronously) interrupted task.
@SamuelAFalvoII @FioraAeterna Right. That's why for async interruption (e.g. timer interrupt), you have to save all. Doesn't apply @ syscall
@SamuelAFalvoII @FioraAeterna Signal handlers only need to faithfully save registers when interrupting asynchronously. This is impl detail.
@RichFelker @FioraAeterna I believe POSIX signal handlers are specified to be asynchronous.
@SamuelAFalvoII @FioraAeterna Yes and no; too subtle for 140 chars. But _when_ they trigger at a syscall, there's nothing to save.
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