On Sun, Dec 2, 2012 at 6:12 PM, Julius Baxter <[email protected]> wrote: > On Sun, Dec 2, 2012 at 9:38 PM, Matthew Hicks <[email protected]> wrote: >> On Sun, Dec 2, 2012 at 12:11 PM, Julius Baxter <[email protected]> >> wrote: >>> Hi all >>> >>> I'm preparing what will hopefully be a pretty final draft for the 1.0 >>> spec release. I have come up with a few more points I wouldn't mind >>> getting comment on. >>> >>> 1. Accessing SPRs with insufficient privileges >>> >>> I reckon accessing SPRs which are only accessible in supervisor mode, >>> while in user mode, should basically do nothing - writing should >>> behave like a l.nop and reading should return zero, as if the SPR is >>> unimplemented. >> >> This is a bad idea. You want to make the processor fully >> virtualizable. This requires the ability for a hypervisor to >> interpose on an unmodified operating system. For this to work, the >> hypervisor changes the operating system privilege to 0 (user mode) and >> when the operating system accesses or modifies privileged state, an >> exception triggers and the hypervisor takes over. With your proposal, >> the hypervisor would never know and the system would collapse. >> >> I suggest using a illegal instruction exception. In general, I am not >> of fan of hardware imposed failure oblivious computing. That is a >> choice software should make. > > Hi Matthew, > > I am a fan of getting this architecture spec update done, and it looks > like this particular suggestion has resulted in more questions than > answers. It seems that we could have a useful feature such as trapping > all unprivileged accesses to SPRs, but I'd rather not delay this set > of updates for us to flesh it out, because it's not critical. A lot of > the other fixes, however, are. So here's what I propose - we leave it > out of this draft, but perhaps mention that it is undefined and > implementation-dependent. At a later point, we can easily define the > sort of behaviour you suggest once it's been fully thought through and > we're convinced there's a need for this. > > Cheers > > Julius
What is the rush? I think it is better to take the time to do it in a well thought out manner the first time, then to do it quickly and later have to add a special SPR bit somewhere that requires both hardware and software support to accommodate systems written to the first, hastily done spec. ---Matthew Hicks _______________________________________________ OpenRISC mailing list [email protected] http://lists.openrisc.net/listinfo/openrisc
