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Message-ID: <1518627219.12890.150.camel@infradead.org>
Subject: Re: [PATCH 1/4] x86/speculation: Use IBRS if available before
 calling into firmware
From:   David Woodhouse <dwmw2@infradead.org>
To:     Tom Lendacky <thomas.lendacky@amd.com>, tglx@linutronix.de,
        karahmed@amazon.de, x86@kernel.org, kvm@vger.kernel.org,
        torvalds@linux-foundation.org, pbonzini@redhat.com,
        linux-kernel@vger.kernel.org, bp@alien8.de, peterz@infradead.org,
        jmattson@google.com, rkrcmar@redhat.com,
        arjan.van.de.ven@intel.com, dave.hansen@intel.com, mingo@kernel.org
In-Reply-To: <33d227ac-7723-5e5f-c299-4a44daa427c6@amd.com>
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         <33d227ac-7723-5e5f-c299-4a44daa427c6@amd.com>
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Date:   Wed, 14 Feb 2018 16:53:39 +0000
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On Wed, 2018-02-14 at 10:36 -0600, Tom Lendacky wrote:
> On 2/14/2018 10:11 AM, David Woodhouse wrote:
> >=20
> >=20
> >=20
> > On Wed, 2018-02-14 at 10:07 -0600, Tom Lendacky wrote:
> > >=20
> > > Shouldn't these writes to the MSR be just for the IBRS bit?=C2=A0 The=
 spec
> > > also defines the STIBP bit for this MSR, and if that bit had been set=
 by
> > > BIOS for example, these writes will clear it.=C2=A0 And who knows wha=
t future
> > > bits may be defined and how they'll be used.
> >
> > We don't use STIBP. If one day we do decide to set it in userspace for
>
> Right, I understand the kernel doesn't use STIBP, that's why I mentioned
> BIOS as an example.

BIOS has no business setting this for us either. Either we support it
and turn it on in the kernel ourselves, or it's off.
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