spapr: Enable DD2.3 accelerated count cache flush in pseries-5.0 machine

spapr: Enable DD2.3 accelerated count cache flush in pseries-5.0 machine

For POWER9 DD2.2 cpus, the best current Spectre v2 indirect branch
mitigation is "count cache disabled", which is configured with:
    -machine cap-ibs=fixed-ccd
However, this option isn't available on DD2.3 CPUs with KVM, because they
don't have the count cache disabled.

For POWER9 DD2.3 cpus, it is "count cache flush with assist", configured
with:
    -machine cap-ibs=workaround,cap-ccf-assist=on
However this option isn't available on DD2.2 CPUs with KVM, because they
don't have the special CCF assist instruction this relies on.

On current machine types, we default to "count cache flush w/o assist",
that is:
    -machine cap-ibs=workaround,cap-ccf-assist=off
This runs, with mitigation on both DD2.2 and DD2.3 host cpus, but has a
fairly significant performance impact.

It turns out we can do better.  The special instruction that CCF assist
uses to trigger a count cache flush is a no-op on earlier CPUs, rather than
trapping or causing other badness.  It doesn't, of itself, implement the
mitigation, but *if* we have count-cache-disabled, then the count cache
flush is unnecessary, and so using the count cache flush mitigation is
harmless.

Therefore for the new pseries-5.0 machine type, enable cap-ccf-assist by
default.  Along with that, suppress throwing an error if cap-ccf-assist
is selected but KVM doesn't support it, as long as KVM *is* giving us
count-cache-disabled.  To allow TCG to work out of the box, even though it
doesn't implement the ccf flush assist, downgrade the error in that case to
a warning.  This matches several Spectre mitigations where we allow TCG
to operate for debugging, since we don't really make guarantees about TCG
security properties anyway.

While we're there, make the TCG warning for this case match that for other
mitigations.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>