On Tue, 14 Jan 2025 15:31:14 +0100 Jiri Olsa <olsajiri@xxxxxxxxx> wrote: > > IIRC this is the magic recipe blessed by both Intel and AMD, and > > if we're going to be changing this I would want both vendors to sign off > > on that. > > ok Right. In fact Intel wouldn't sign off on this recipe for a few years. We actually added to the kernel before they gave their full blessing. I got a "wink, it should work" from them but they wouldn't officially say so ;-) But a lot of it has to do with all the magic of the CPU. They have always allowed writing the one byte int3. I figured, if I could write that one byte int3 then run a sync on all CPUs where all CPUs see that change, then nothing should ever care about the other 4 bytes after that int3 (a sync was already done). Then change the 4 bytes and sync again. I doubt the int3 plus the 4 byte change would work, as was mentioned if the other 4 bytes were on another cache line, another CPU could read the first set of bytes without the int3 and the second set of bytes with the update and go boom! This dance was to make sure everything sees everything properly. I gave a talk about this at Kernel-Recipes in 2019: https://www.slideshare.net/slideshow/kernel-recipes-2019-ftrace-where-modifying-a-running-kernel-all-started/177509633#44 -- Steve
