linux-kernel - [PATCH 0/1] x86/speculation: Allow IBPB to be conditionally enabled on CPUs with always-on STIBP

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Date:   Thu, 29 Oct 2020 17:51:32 +1100
From:   Anand K Mistry <amistry@...gle.com>
To:     x86@...nel.org, linux-kernel@...nel.org
Cc:     Thomas.Lendacky@....com, joelaf@...gle.com,
        asteinhauser@...gle.com, tglx@...utronix.de,
        Anand K Mistry <amistry@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Mike Rapoport <rppt@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Vineela Tummalapalli <vineela.tummalapalli@...el.com>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 0/1] x86/speculation: Allow IBPB to be conditionally enabled
 on CPUs with always-on STIBP

When attempting to do some performance testing of IBPB on and AMD
platform, I noticed the IBPB instruction was never being issued, even
though it was conditionally on and various seccomp protected processes
were force enabling it. Turns out, on those AMD CPUs, STIBP is set to
always-on and this was causing an early-out on the prctl() which turns
off IB speculation. Here is my attempt to fix it.

I'm hoping someone that understands this better than me can explain why
I'm wrong.


Anand K Mistry (1):
  x86/speculation: Allow IBPB to be conditionally enabled on CPUs with
    always-on STIBP

 arch/x86/kernel/cpu/bugs.c | 41 +++++++++++++++++++++-----------------
 1 file changed, 23 insertions(+), 18 deletions(-)

-- 
2.29.1.341.ge80a0c044ae-goog