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

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Message-ID: <660da944-6e24-5ed6-6d85-94efd67180c9@redhat.com>
Date:   Wed, 10 Jan 2018 16:56:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "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 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...

Paolo