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Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation
From:   David Woodhouse <dwmw2@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
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        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>,
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        the arch/x86 maintainers <x86@kernel.org>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>
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Date:   Tue, 06 Feb 2018 09:14:48 +0000
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On Sun, 2018-02-04 at 19:43 +0100, Thomas Gleixner wrote:
> Yet another possibility is to avoid the function entry and accouting magi=
c
> and use the generic gcc return thunk:
>=20
> __x86_return_thunk:
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0call L2
> L1:
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0pause
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0lfence
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0jmp L1
> L2:
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0lea 8(%rsp), %rsp|lea 4(%=
esp), %esp
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ret
>=20
> which basically refills the RSB on every return. That can be inline or
> extern, but in both cases we should be able to patch it out.
>=20
> I have no idea how that affects performance, but it might be worthwhile t=
o
> experiment with that.

That was what I had in mind when I asked HJ to add -mfunction-return.

I suspect the performance hit would be significant because it would
cause a prediction miss on *every* return.

But as I said, let's implement what we can without IBRS for Skylake,
then we can compare the two options for performance, security coverage
and general fugliness.
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