It's an instruction in the sense that timing boundaries are x86 instruction boundaries, which is what the security model bases itself on. So yeah, not an instruction in the strict CPU sense (microcode + micro-ops), but in the useful sense.
replyA read that happens to touch a particular torment nexus fd is still a long-running syscall, even if the syscall servicing routine itself is not long-running. The underlying problem is that program code that is "in a syscall" or "in an instruction" is in a special state for which interruption might not be possible or implemented well[0].
reply[0] Remember ITS and the PC2 problem?
For other people who didn't immediately recognize "ITS and the PC2 problem" and want the backstory it starts in the last paragraph of page 312: https://web.mit.edu/~simsong/www/ugh.pdf
replyI have a reproducible way to have a pwrite syscall on a specific SSD on a specific machine take 15+ seconds and completely block any syscall related to that SSD by any other thread or core during that amount of time. I tried and couldn't preempt it either (sched_fifo and preempt kernel options). I should have a look soon with Intel PT to check whether it's on the same instruction every time :)
replyOh, it totally is, it just doesn't prompt the same sense of glee and wonderment.
replySo we’re in agreement
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