On Mon, 03 May 2021 20:12:21 +0100, Ricardo Koller <ricarkol@xxxxxxxxxx> wrote: > > On Mon, May 03, 2021 at 11:32:39AM +0100, Marc Zyngier wrote: > > On Sat, 01 May 2021 00:24:06 +0100, > > Ricardo Koller <ricarkol@xxxxxxxxxx> wrote: [...] > > > + .if \vector >= 8 > > > + mrs x1, sp_el0 > > > > I'm still a bit perplexed by this. SP_EL0 is never changed, since you > > always run in handler mode. Therefore, saving/restoring it is only > > overhead. If an exception handler wants to introspect it, it is > > already available in the relevant system register. > > > > Or did you have something else in mind for it? > > > > Not really. The reason for saving sp_el0 in there was just for > consistency, so that handlers for both el0 and el1 exceptions could > get the sp at regs->sp. We already have sp_el0 consistency by virtue of having it stored in in a sysreg. > Restoring sp_el0 might be too much. So, what do you think of this > v3: we keep the saving of sp_el0 into regs->sp (to keep things the > same between el0 and el1) and delete the restoring of sp_el0? To me, the whole purpose of saving some some context is to allow the exception handling code to run C code and introspect the interrupted state. But saving things that are not affected by the context change seems a bit pointless. One thing I'd like to see though is to save sp_el1 as it was at the point of the exception (because that is meaningful to get the exception context -- think of an unaligned EL1 stack for example), which means correcting the value that gets saved. So I would suggest to *only* save sp_el1, to always save it (irrespective of the exception coming from EL0 or EL1), and to save a retro-corrected value so that the handler can directly know where the previous stack pointer was. Thanks, M. -- Without deviation from the norm, progress is not possible.
