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

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Date:   Wed, 10 Jan 2018 16:05:13 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "arjan@...ux.intel.com" <arjan@...ux.intel.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "liran.alon@...cle.com" <liran.alon@...cle.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On Wed, 2018-01-10 at 16:56 +0100, Paolo Bonzini wrote:
> On 10/01/2018 16:48, Woodhouse, David wrote:
> >>
> >> And what is the point of this "always set IBRS=1" then? Are there
> >> some other things lurking in the shadows?
> > Yes. *FUTURE* CPUs will have a mode where you can just set IBRS and
> > leave it set for ever and not worry about any of this, and the
> > performance won't even suck.
> > 
> > Quite why it's still an option you have to set in an MSR, and not just
> > a feature bit that they advertise and do it unconditionally, I have no
> > idea. But apparently that's the plan.
> 
> And again---why you still need IBPBs.  That also escapes me.  I wouldn't
> be surprised if that's just a trick to sneak it in a generation earlier...

There was some suggestion in early development that guests/processes
with a different CR3 or different VMCS would be considered "mutually
less privileged". It got dropped, and I'm sure we'll have clarification
before IBRS_ATT actually gets released.

I think the current docs do suggest that you need IBPB on context
switch (and vmptrld) still. But I expect that *might* go away if we're
lucky. And really, we have other things to work on for now before we
support hypothetical future hardware :)
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