linux-kernel - Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Message-ID: <20180110154138.GE29272@char.us.oracle.com>
Date:   Wed, 10 Jan 2018 10:41:38 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Liran Alon <liran.alon@...cle.com>, jmattson@...gle.com,
        x86@...nel.org, dwmw@...zon.co.uk, bp@...en8.de,
        aliguori@...zon.com, thomas.lendacky@....com, rkrcmar@...hat.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On Wed, Jan 10, 2018 at 03:28:43PM +0100, Paolo Bonzini wrote:
> On 10/01/2018 15:06, Arjan van de Ven wrote:
> > On 1/10/2018 5:20 AM, Paolo Bonzini wrote:
> >> * a simple specification that does "IBRS=1 blocks indirect branch
> >> prediction altogether" would actually satisfy the specification just as
> >> well, and it would be nice to know if that's what the processor actually
> >> does.
> > 
> > it doesn't exactly, not for all.
> > 
> > so you really do need to write ibrs again.
> 
> Okay, so "always set IBRS=1" does *not* protect against variant 2.  Thanks,

And what is the point of this "always set IBRS=1" then? Are there some other
things lurking in the shadows?
> 
> Paolo