Not really userspace

Posted Jul 18, 2024 9:40 UTC (Thu) by 4lDO2 (guest, #172237)
In reply to: Not really userspace by Cyberax
Parent article: Redox to implement POSIX signals in user space

A SIGSEGV signal, which on Redox will be handled as an 'exception' that userspace can later call a signal handler for (basically the same type of jump, passing the page fault address, old instr pointer, page fault flags, etc), does *not* necessarily require several context switches. In fact, it can in theory be just as fast as a kernel-corrected page fault, which should be slightly but not significantly slower than a syscall, i.e. two mode switches. It wouldn't surprise me if Linux's implementation is not as efficient though.

The point of catching SIGSEGV this way, is not that exceptions are faster than checks, but that they *are* faster overall if the probability of the check failing, is low enough to justify avoiding this check in the general case. After all, this is why Linux (and Redox) implements copy_from_user as a catchable memcpy function, rather than walking the page tables every time. Most applications won't EFAULT more than once.

Not really userspace

Posted Jul 18, 2024 19:30 UTC (Thu) by Cyberax (✭ supporter ✭, #52523) [Link] (1 responses)

> A SIGSEGV signal, which on Redox will be handled as an 'exception' that userspace can later call a signal handler

FWIW, Windows also uses a similar model. Memory faults can be caught via SEH ("Structured Exception Handling"). It still has to round-trip through the kernel, but it's more lightweight compared to signals.

> The point of catching SIGSEGV this way, is not that exceptions are faster than checks, but that they *are* faster overall if the probability of the check failing, is low enough to justify avoiding this check in the general case. After all, this is why Linux (and Redox) implements copy_from_user as a catchable memcpy function, rather than walking the page tables every time. Most applications won't EFAULT more than once.

Sure, but then it's also fine if the signal handling is done through a userland process/thread. It will add a couple of context switches, but it'll still be fast enough.

Not really userspace

Posted Jul 19, 2024 16:16 UTC (Fri) by 4lDO2 (guest, #172237) [Link]

> FWIW, Windows also uses a similar model. Memory faults can be caught via SEH ("Structured Exception Handling"). It still has to round-trip through the kernel, but it's more lightweight compared to signals.

That's pretty cool. My understanding is that Windows generally offloads much more logic to userspace, as they can freely change the kernel/user ABI.

> Sure, but then it's also fine if the signal handling is done through a userland process/thread. It will add a couple of context switches, but it'll still be fast enough.

Yeah probably. Redox will most likely implement userspace exception handling synchronously, like signals, but for many workloads those edge cases would presumably not be that noticeable either way.