On Wed, Aug 14, 2019 at 08:46:15PM +1000, Michael Ellerman wrote: > Claudio Carvalho <cclau...@linux.ibm.com> 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 ยง 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
