arm64: handle watchpoint exceptions from EL0

[mhorne]

Test Plan

Triggering an EL0 watchpoint generates SIGTRAP after this patch.

[mgorny_gentoo.org]

sys/arm64/arm64/trap.c

539

I'm not sure here and I can't figure out the abstractions in Linux code but on Linux I'm getting a SIGTRAP with si_addr pointing to the watched memory rather than the instruction, and LLDB uses that to identify which watchpoint was hit. Do you have any suggestion if I can determine that any other way?

[mhorne]

sys/arm64/arm64/trap.c

539

Hmm. Returning the watched address in si_addr does seem more useful. I don't think there is any other simple way to determine which watchpoint triggered the exception, so we can probably switch over to this.

My only reservation is that FreeBSD/amd64 does not appear to behave this way, and it returns the trapping instruction address in si_addr.

[mgorny_gentoo.org]

sys/arm64/arm64/trap.c

539

Indeed it doesn't. We are working around that by checking DR7 directly.

[mhorne]

Pass the value of the Fault Address Register to si_addr. This contains the address that triggered the watchpoint.

[mgorny_gentoo.org]

Thanks, I was able to get working hardware watchpoints with this patch.

[mhorne]

Update siginfo(3) man page to note the differing behaviour of si_addr.

[jhb]

share/man/man3/siginfo.3

221–224

I'm not sure I would keep the arm64 sentence as we don't document the MD behaviors for other signals here but instead just use "may" to indicate it can vary.