https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #18 from Bill Schmidt ---
I asked around a bit. On x86, user-user attacks are not mitigated by default.
To enable user-user mitigation:
echo 2 > /sys/kernel/debug/x86/ibrs_enabled
My source tells me:
8<---
Red Hat
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #17 from Bill Schmidt ---
OK, thanks! I'd be very interested in hearing what you discover.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #16 from Timothy Pearson ---
(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
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #15 from Bill Schmidt ---
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
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #14 from Timothy Pearson ---
(In reply to Bill Schmidt from comment #13)
> This was prototyped and measured against the firmware fixes with
> indistinguishable results. So the complexity of a software solution, with
> its impacts on
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #13 from Bill Schmidt ---
This was prototyped and measured against the firmware fixes with
indistinguishable results. So the complexity of a software solution, with its
impacts on Linux distributions, was not warranted. (That is,
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #12 from Timothy Pearson ---
After quite a bit of investigation, this is down to the Spectre v2 user mode
protections on POWER9, which (from what I understand) involve completely
disabling the branch predictor.
My question then
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #11 from Bill Schmidt ---
(In reply to Timothy Pearson from comment #10)
>
> It's even slow compared to P8 with mitigations applied. Do you have a link
> to the hostboot commit that may have enabled the P9 mitigation, or to the
>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #10 from Timothy Pearson ---
(In reply to Bill Schmidt from comment #9)
> You mentioned you're on a POWER9 machine. It could be that you have
> firmware with Spectre mitigations applied, which will affect all indirect
> branches.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #9 from Bill Schmidt ---
You mentioned you're on a POWER9 machine. It could be that you have firmware
with Spectre mitigations applied, which will affect all indirect branches. It
may be that you do not have Spectre mitigations
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #8 from Richard Biener ---
(In reply to Timothy Pearson from comment #4)
> (In reply to Andrew Pinski from comment #3)
> > This is 100% the equivalent code.
> >
> > jmp *(%r15) # opline.199_67->handler
> > Does two things:
> >
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #7 from David Edelsohn ---
One possibility is bad luck and the branch happens to fall on an address that
conflicts with another branch.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #6 from Timothy Pearson ---
Understood. I'll update this report if we find a way to get the predictor
working optimally in this scenario.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
David Edelsohn changed:
What|Removed |Added
Status|RESOLVED|REOPENED
Resolution|INVALID
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #4 from Timothy Pearson ---
(In reply to Andrew Pinski from comment #3)
> This is 100% the equivalent code.
>
> jmp *(%r15) # opline.199_67->handler
> Does two things:
> loads a pointer from %r15 and then jumps to that pointer.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
Andrew Pinski changed:
What|Removed |Added
Status|WAITING |RESOLVED
Last reconfirmed|2018-04-05
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
--- Comment #2 from Timothy Pearson ---
(In reply to David Edelsohn from comment #1)
> What two additional instructions? x86 is a CISC architecture and Power is a
> RISC architecture. x86 has an instruction that directly performs an
> indirect
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85216
David Edelsohn changed:
What|Removed |Added
Status|UNCONFIRMED |WAITING
Last reconfirmed|
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