[Bug target/85216] Performance issue with PHP on ppc64 systems

[tpearson at raptorengineering dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216

--- Comment #16 from Timothy Pearson <tpearson at raptorengineering dot com> ---
(In reply to Bill Schmidt from comment #15)
> PHP's reliance on frequent indirect branches makes it essentially the worst
> case for this sort of thing.  When Spectre v2 CVE mitigations are in place
> for user code, you will see performance issues on all architectures that
> rely on speculation for indirect branch performance.  When user code is
> running in an "unsafe" configuration, you will not see those issues.  (We
> have seen similar issues on x86 when retpoline is used for user code.)

What's most puzzling is that we're looking at benchmarks on x86 systems that
are supposed to be mitigated, but the performance drop isn't really showing up.
 At this point I'm wondering if:

a.) The user/user attack isn't actually mitigated on these systems, only the
user/kernel attack

b.) Intel/AMD found some way to update the microcode so as not to have a heavy
performance loss

In any case, we'll continue to investigate / run benchmarks to see if any light
can be shed on this.