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Message-ID: <1516874209.30244.38.camel@infradead.org>
Subject: Re: [PATCH v4 5/7] x86/pti: Do not enable PTI on processors which
 are not vulnerable to Meltdown
From:   David Woodhouse <dwmw2@infradead.org>
To:     Peter Zijlstra <peterz@infradead.org>
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, bp@alien8.de, pbonzini@redhat.com,
        ak@linux.intel.com, torvalds@linux-foundation.org,
        gregkh@linux-foundation.org, dave.hansen@intel.com,
        gnomes@lxorguk.ukuu.org.uk, ashok.raj@intel.com, mingo@kernel.org
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Date:   Thu, 25 Jan 2018 09:56:49 +0000
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On Thu, 2018-01-25 at 10:42 +0100, Peter Zijlstra wrote:
> On Thu, Jan 25, 2018 at 09:23:07AM +0000, David Woodhouse wrote:
> > +static bool __init early_cpu_vulnerable_meltdown(struct cpuinfo_x86 *c=
)
> > +{
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0u64 ia32_cap =3D 0;
> > +
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (x86_match_cpu(cpu_no_meltdown))
> > +=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 return false;
> > +
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (cpu_has(c, X86_FEATURE_ARCH_CAPABILI=
TIES))
> > +=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=A0rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
>=20
> I think it was suggested a while back to write this like:
>=20
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (cpu_has(c, X86_FEATUR=
E_ARCH_CAPABILITIES) &&
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 !rdmsr=
l_safe(MSR_IA32_ARCH_CAPABILITIES, ia32_cap))
>=20
> to deal with funny virt scenarios where they accidentally advertise the
> CPUID bit but don't in fact provide the MSR.

It was indeed suggested, but I was a bit confused by that. Because the
CPUID bit exists *purely* to advertise the existence of that MSR;
nothing more.

If it doesn't exist we'll end up with zero in ia32_cap anyway, which
will mean we *won't* see the RDCL_NO bit, and won't disable the
Meltdown flag.
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