| Rich Freeman on 3 Jan 2018 11:00:41 -0800 |
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| Re: [PLUG] The mysterious case of the Linux Page Table Isolation patches |
On Wed, Jan 3, 2018 at 1:37 PM, Ronald Guilmet <ronpguilmet@gmail.com> wrote: > My first thought was that hardware is cheap today. I think CISC has shorting > instructions, but uses more memory. When the hardware (RAM) was the cost of > a car, they pushed the software to take some of the burden, right? > Well, the original driver was hand-written assembly, which is way easier to do with CISC. This is valid on x86: mov edx, [esi+4*ebx]. That is obviously pretty useful if you're working with an array, but obviously it is a bunch of work for the processor to handle every one of those indexing modes, especially since it means that MOV instruction has 14 different opcodes, and I couldn't even quickly find anything on line that indicates how the instruction sizes vary. Granted, to the degree that different opcodes are used to the CPU they're really different instructions. Oh, and I found this gem: https://www.cl.cam.ac.uk/~sd601/papers/mov.pdf And this even bigger gem: https://github.com/xoreaxeaxeax/movfuscator Seriously, thanks for this thread because that last link made my whole day. Maybe memory or memory bandwidth was a benefit, but I have to wonder how much was lost in inefficiencies with the cache/etc since instruction size is variable. -- Rich ___________________________________________________________________________ Philadelphia Linux Users Group -- http://www.phillylinux.org Announcements - http://lists.phillylinux.org/mailman/listinfo/plug-announce General Discussion -- http://lists.phillylinux.org/mailman/listinfo/plug