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Message-ID: <1516756923.13558.62.camel@infradead.org>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation
From:   David Woodhouse <dwmw2@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        kvm list <kvm@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>
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         <4e01a7a9-29e4-adcc-3f53-550fb7f3d370@amd.com>
         <1516724457.9521.156.camel@amazon.co.uk>
         <fb2b5589-e14c-470e-2de0-d9da1667bace@amd.com>
         <20180123224956.GQ7844@tassilo.jf.intel.com>
         <1516749276.13558.25.camel@amazon.co.uk>
         <cac69284-e76f-6081-62db-adb4dc123045@linux.intel.com>
         <CALCETrUTjw+KHFA3a4eL+ds=HOoK_BFWZPdkeOkfcn=Oq-F2+w@mail.gmail.com>
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Date:   Wed, 24 Jan 2018 01:22:03 +0000
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On Tue, 2018-01-23 at 17:00 -0800, Andy Lutomirski wrote:
> On Tue, Jan 23, 2018 at 4:47 PM, Tim Chen <tim.c.chen@linux.intel.com> wr=
ote:
> >=20
> > On 01/23/2018 03:14 PM, Woodhouse, David wrote:
> > >=20
> > > On Tue, 2018-01-23 at 14:49 -0800, Andi Kleen wrote:
> > > >=20
> > > > >=20
> > > > > Not sure.=C2=A0=C2=A0Maybe to start, the answer might be to allow=
 it to be set for
> > > > > the ultra-paranoid, but in general don't enable it by default.=C2=
=A0=C2=A0Having it
> > > > > enabled would be an alternative to someone deciding to disable SM=
T, since
> > > > > that would have even more of a performance impact.
> > > > I agree. A reasonable strategy would be to only enable it for
> > > > processes that have dumpable disabled. This should be already set f=
or
> > > > high value processes like GPG, and allows others to opt-in if
> > > > they need to.
> > > That seems to make sense, and I think was the solution we were
> > > approaching for IBPB on context switch too, right?
> > >=20
> > > Are we generally agreed on dumpable as the criterion for both of thos=
e?
> > >=20
> > It is a reasonable approach.=C2=A0=C2=A0Let a process who needs max sec=
urity
> > opt in with disabled dumpable. It can have a flush with IBPB clear befo=
re
> > starting to run, and have STIBP set while running.
> >=20
> Do we maybe want a separate opt in?=C2=A0=C2=A0I can easily imagine thing=
s like
> web browsers that *don't* want to be non-dumpable but do want this
> opt-in.
=C2=A0
This is to protect you from another local process running on a HT
sibling. Not the kind of thing that web browsers are normally worrying
about.

> Also, what's the performance hit of STIBP?

Varies per CPU generation, but generally approaching that of full IBRS
I think? I don't recall looking at this specifically (since we haven't
actually used it for this yet).
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