Hi Maciej, > This change makes no sense to me anyway I am afraid. > > At the error level (Status.ERL=1) the user segment becomes unmapped and > therefore all KUSEG addresses become physical addresses. Which means that > if any of this code you have patched is called to access user pages, then > you have a bigger problem than just the cache going out of sync. > > The only reason to access KUSEG at the error level is to save/restore > register state to/from a dedicated RAM area offset from $zero so that > execution is restartable. Unlike at the exception level you cannot use > $k0 and $k1 as temporaries, because an error exception can happen any time > including in particular while $k0 and $k1 are in active use at the > exception level, so clobbering them would make the system non-restartable > (of course receiving an error exception may mean that anyway). > > Code to write/read that dedicated area should be purpose-crafted and the > area won't be accessed at any other time, so the issue of being cache > coherent or not does not apply as the area will never be accessed with > caching operations. Thanks for your detailed description! > I can see the R5900 has additional classes of error exceptions defined, > such as debug and performance counter exceptions, which are not related to > hardware faults and can happen in regular execution in response to certain > conditions requested. If you want to handle these implementation specific > extensions and consequently serve these exceptions, then please take care > of all the requirements as code to support them is added. Yes, hardware breakpoints and hardware performance counters are very interesting features to develop after the initial submission. > Though as I wrote above it does not look to me like anything specific > will be needed -- the handler at entry will save the state necessary for > restartability to a dedicated RAM area first and then to the kernel stack, > switch the error level off, do the necessary processing, and then reverse > the steps before resuming execution interrupted. Excellent. I will just drop this patch then. Fredrik
