Excerpts from Segher Boessenkool's message of May 6, 2020 8:11 am:

Hi!

On Thu, Apr 30, 2020 at 02:02:02PM +1000, Nicholas Piggin wrote:

quoted

Add support for the scv instruction on POWER9 and later CPUs.

Looks good to me in general :-)

Thanks for taking a look.

quoted

For now this implements the zeroth scv vector 'scv 0', as identical
to 'sc' system calls, with the exception that lr is not preserved, and
it is 64-bit only. There may yet be changes made to this ABI, so it's
for testing only.

What does it do with SF=0?  I don't see how it is obviously not a
security hole currently (but I didn't look too closely).

Oh that's an outdated comment, I since decided better to keep all the code 
common and handle 32-bit compat the same way as existing sc syscall.

Thanks,
Nick