On 8 July 2010 16:00, Paul Gilmartin <[email protected]> wrote: > On 07/08/10 13:56, Paul Raulerson wrote: >> >> Nope- the machine itself will enforce execution control of the >> instructions. There are various combinations of bit settings in the PSW and >> or control registers that control >> whether the "semi-privileged" instructions can execute or not. >> >> >> This has been around a long time, and was (probably) used in VM more >> extensively than OS/390 or z/OS. I think most of the instructions involved >> control program flow >> or memory usage, but don't take that as gospel. >> >> > There's also SIE. I don't know whether Linux uses this; I'm confident that VM > does.
This is really getting far from the original topic, which was a completely UNprivileged instruction executing under Linux. The claim was made that Linux can somehow "restrict" use of these instructions, and that is simply not true. In S/370 architecture there were no semi-privileged instructions, and no SIE. No operating system had any way of "intercepting" or suppressing or otherwise affecting the execution of an unprivileged instruction, and this remains true today. In 370/XA there are no semi-privileged instructions, but VM was implemented using SIE, and this allowed for the later introduction of semi-privileged instructions with acceptable performance. In newer architectures (370/ESA and above) there are semi-privileged instructions, and their use in a virtual machine is managed by SIE. Current versions of VM are largely OCO, so I don't know how they use the various available controls over semi-privileged instructions, but I strongly suspect they use SIE as the only such control mechanism. Tony H.
