Re: [PATCH 07/13] KVM: PPC: Book3S HV: Adjust host/guest context switch for POWER9

[Paul Mackerras <paulus@ozlabs.org>]

On Fri, Nov 18, 2016 at 08:05:47PM +0530, Aneesh Kumar K.V wrote:
> Paul Mackerras <paulus@ozlabs.org> writes:
> 
> > Some special-purpose registers that were present and accessible
> > by guests on POWER8 no longer exist on POWER9, so this adds
> > feature sections to ensure that we don't try to context-switch
> > them when going into or out of a guest on POWER9.  These are
> > all relatively obscure, rarely-used registers, but we had to
> > context-switch them on POWER8 to avoid creating a covert channel.
> > They are: SPMC1, SPMC2, MMCRS, CSIGR, TACR, TCSCR, and ACOP.
> 
> We don't need to context-switch them even when running a power8 compat
> guest ?

They physically don't exist on the P9 chip, so how could we
context-switch them?  They certainly can't be used as a covert
channel.

Accesses to them will be a no-op for the guest in privileged
(supervisor) mode (i.e., mfspr won't modify the destination
register), which could be confusing for the guest if it was expecting
to use them.  SPMC1/2 and MMCRS are part of the "supervisor" PMU,
which we have never used.  I think CSIGR, TACR and TCSCR are part of a
facility that was never completely implemented or usable on P8, so
nothing uses them.  ACOP is used in arch/powerpc/mm/icswx.c in
conjunction with accelerators.  There might be a problem there, but in
any case, with no physical ACOP register present there's no way to
save/restore it.

Paul.