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Monday, 11 October 2021

00:17:00 <geist> around the freenode collapse i went ahead and fully switched to irccloud and i've been pretty happy with
00:17:00 <geist> but it's not free, though it's also not expensive
00:32:00 <clever> i already had an old gentoo box dedicated to just running irssi under screen
09:15:00 <froggey> does anyone know how much overhead running with alignment checking enabled has on x86? (ie EFLAGS.AC=1)
09:18:00 <zid> I am assuming none.. unless you take the exception
09:18:00 <moon-child> froggey: based purely on the fact that e.g. movaps and movups are the same speed for aligned data, I would assume that it _can_ be free. Whether they actually bothered to make it so, I have no idea
09:18:00 <zid> yea, conceptually it's simple
09:19:00 <zid> check the bottom 2 bits of the address, signal based on some and gate or whatever
09:19:00 <zid> so if it's slow.. they're being mean
09:20:00 <moon-child> zid: well, you have to roll back the state to the point where the fault happens. Cuz superscalar. Already have to do that for e.g. bad perms, bad decode, branch mispred, etc, so the infra is there for it, but this adds another case you have to handle; they may not have considered it worhtwhile
09:20:00 <zid> the fault being slow is fine
09:22:00 <moon-child> yes, my point is you have to keep enough state around to roll back when you perform the fault
09:22:00 <gog> seems like more hassle than just doing the unaligned access
09:22:00 <froggey> yeah, I was wondering about normal operation. I don't care if the fault is slow
09:23:00 <zid> you can't enable it on windows, windows is naughty
09:23:00 <zid> and dies
16:20:00 <Qubasa> Does someone here know how to get "perf kvm" to read and apply my dwarf/ELF symbols?
16:23:00 <Qubasa> I tried "nm kernel.elf > kernel.map" and then "sudo perf kvm --guestkallsyms=kernel.map record"
23:38:00 <geist> good question. Honestly i've never used the 'kvm' command, which i guess is a wrapper around libvirt or whatnot?
23:38:00 <geist> I always drive qemu directly
23:40:00 <gog> don't drink and qemu
23:47:00 <klange> I think it's just a wrapper around qemu directly that ensures -enable-kvm
23:48:00 <klange> #! /bin/sh
23:48:00 <klange> exec qemu-system-x86_64 -enable-kvm "$@"
23:48:00 <klange> It is exactly that.