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Message-ID: <1516560862.9814.47.camel@infradead.org>
Subject: Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB
From:   David Woodhouse <dwmw2@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     arjan@linux.intel.com, tglx@linutronix.de, karahmed@amazon.de,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        tim.c.chen@linux.intel.com, peterz@infradead.org,
        pbonzini@redhat.com, ak@linux.intel.com,
        torvalds@linux-foundation.org, gregkh@linux-foundation.org
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Date:   Sun, 21 Jan 2018 18:54:22 +0000
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On Sun, 2018-01-21 at 19:06 +0100, Borislav Petkov wrote:
>=20
> > switch to using ALTERNATIVES instead of static_cpu_has]
>=20
> Why?
>=20
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (static_cpu_has(X86_FE=
ATURE_IBPB))
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0wrmsr(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, 0);
>=20
> It can't get any more readable than this. Why even f*ck with
> alternatives?

Because we're backporting this to every stable kernel under the sun,
and they don't already require asm-goto. Peter does have a dozen or so
objtool patches to protect us against the missed GCC optimisation which
would make it vulnerable via a conditional branch, but we'll do that
*after* the basic backportable implementation using ALTERNATIVE goes
in.
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