Re: [PATCH 2/4] target/ppc: Add recording of taken branches to BHRB

[Glenn Miles]

> > It all looks pretty good otherwise. I do worry about POWER8/9 which
> > do not have BHRB disable bit. How much do they slow down I wonder?
>
> That is a good question!  I'll see if I can get some linux boot times
> with and without the changes on P9.

I ran some tests booting ubuntu 20.04 on the powrnv9 model and found
that it took about 5% longer to boot to the login prompt with the
changes than without.  As discussed offline, I'll remove this support
from P8/P9 as the 5% reduction in performance is not worth the added
functionality.  We could perhaps add it back in the future with a
command line option that has it disabled by default on P8/P9.

P10 and on has it disabled for non problem state branches, so we
should be good there.

Thanks,

Glenn