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Message-ID: <1516565226.9814.63.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 20:07:06 +0000
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On Sun, 2018-01-21 at 20:54 +0100, Borislav Petkov wrote:
> On Sun, Jan 21, 2018 at 07:31:39PM +0000, David Woodhouse wrote:
> > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (boot_cpu_has(X86_=
FEATURE_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);
> > >=C2=A0
> > > Problem solved.
> >=C2=A0
> > Nope. Plenty of patch sets *did* have the bug above though, until it
> > was spotted.
>=20
> And that bug is...?

That bug is the *reason* we're arguing about static_cpu_has vs.
ALTERNATIVE.=C2=A0

A conditional branch that the CPU sees can be speculated over...

Now, Andrew is right that in a number of cases there will be another
serialising instruction before we ever hit a problematic indirect
branch. But as I just said elsewhere, I'd really like the *primitives*
to support unconditional operation.
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