ca125f9c17
When interrupting a kernel context, we don't need to switch stacks (memory nor register). As such, we were also not restoring the register stack pointer (ar.bspstore). This, however, fails to be valid in 1 situation: when we interrupt a register stack switch as is being done in restorectx(). The problem is that restorectx() needs to have ar.bsp == ar.bspstore before it can assign the new value to ar.bspstore. This is achieved by doing a loadrs prior to assigning to ar.bspstore. If we take an interrupt in between the loadrs and the assignment and we don't make sure we restore the ar.bspstore prior to returning from the interrupt, we switch stacks with possibly non-zero dirty registers, which means that the new frame pointer (ar.bsp) will be invalid. So, instead of jumping over the restoration of the register frame pointer and related registers, we conditionalize it based on whether we return to kernel context or user context. A future performance tweak is possible by only restoring ar.bspstore when returning to kernel mode *and* when the RSE is in enforced lazy mode. One cannot assume ar.bsp == ar.bspstore if the RSE is not in enforced lazy mode anyway. While here (well, not quite) don't unconditionally assign to ar.bspstore in exception_save. Only do that when we actually switch stacks. It can only harm us to do it unconditionally. Approved by: re@ (blanket)
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