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Date:   Mon, 26 Nov 2018 21:58:27 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andrea Arcangeli <aarcange@redhat.com>
cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
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        Andrew Lutomirski <luto@kernel.org>,
        Jiri Kosina <jkosina@suse.cz>, thomas.lendacky@amd.com,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>, dave.hansen@intel.com,
        Casey Schaufler <casey.schaufler@intel.com>,
        "Mallick, Asit K" <asit.k.mallick@intel.com>,
        "Van De Ven, Arjan" <arjan@linux.intel.com>, jcm@redhat.com,
        longman9394@gmail.com, Greg KH <gregkh@linuxfoundation.org>,
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Subject: Re: [patch V2 27/28] x86/speculation: Add seccomp Spectre v2 user
 space protection mode
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On Mon, 26 Nov 2018, Andrea Arcangeli wrote:

> Hello,
> 
> On Sun, Nov 25, 2018 at 11:28:59PM +0100, Thomas Gleixner wrote:
> > Indeed. Just checked the documentation again, it's also not clear whether
> > IBPB is required if STIPB is in use.
> 
> I tried to ask this question too earlier:
> 
> https://lkml.kernel.org/r/20181119234528.GJ29258@redhat.com
> 
> If the BTB mistraining in SECCOMP context with STIBP set in SPEC_CTRL,
> can still influence the hyperthreading sibling after STIBP is cleared,
> IBPB is needed before clearing STIBP. Otherwise it's not. Unless told
> otherwise, it'd be safe to assume IBPB is needed in such case.

IBPB is still issued. I won't change that before we have clarification.

But I doubt it's necessary. STIBP seems to be a rather big hammer.

Thanks,

	tglx