On Wed, Aug 14, 2019 at 08:46:15PM +1000, Michael Ellerman wrote: > Claudio Carvalho <cclaudio@xxxxxxxxxxxxx> writes: > > +_GLOBAL(ucall_norets) > > +EXPORT_SYMBOL_GPL(ucall_norets) > > + mfcr r0 > > + stw r0,8(r1) > > + > > + sc 2 /* Invoke the ultravisor */ > > + > > + lwz r0,8(r1) > > + mtcrf 0xff,r0 > > + blr /* Return r3 = status */ > > Paulus points that we shouldn't need to save CR here. Our caller will > have already saved it if it needed to, and we don't use CR in this > function so we don't need to save it. > > That's assuming the Ultravisor follows the hcall ABI in which CR2-4 are > non-volatile (PAPR § 14.5.3). And assuming the ultravisor already clears (or sets, or whatever) all CR fields it does not want to leak the contents of (which it also should, of course). > I know plpar_hcall_norets() does save CR, but it shouldn't need to, that > seems to be historical. aka. no one knows why it does it but it always > has. Segher
