On Thu, 6 Oct 2016, James Hogan wrote: > > How can we install a handler then when we don't know what the upper 32 > > bits of EBase are? > > Right now its assumed the default upper 32 bits are sign extension of > bit 31 in that case (i.e. thats what upper 32bits are clobbered to). I > think the only case where that might not be true would be where WG is > implemented and the bootloader has changed them to e.g. somewhere in > XKPhys, and then cleared WG. We could catch that most of the time by > detecting changed bits 31:30 (as I think you suggested before), but it > still isn't watertight (e.g. xkphys+0x80000000), so if in doubt we > should probably be sure to allocate our own exception vector instead of > guessing at the boot one. What a mess :-(. Does it really matter in reality though? By keeping EBase unchanged we try to install exception handlers in memory assigned by the firmware. This may not necessarily be safe. I think we actually ought to set EBase ourselves, perhaps on a CPU by CPU basis in an MP system, pointing to memory we know we can use at will. If this is going to consume say a page of memory per CPU, then still I don't think it's a huge waste, and any firmware memory safe to reclaim after boostrap we can use for other purposes. Have I missed anything? Maciej
