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Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to
 MSR_IA32_SPEC_CTRL
To:     Jim Mattson <jmattson@google.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <b7039ff3-72f9-92c5-747b-4fba4d578ac5@redhat.com>
Date:   Wed, 31 Jan 2018 16:59:10 -0500
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On 31/01/2018 16:53, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 1:42 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
>> Can we just say it sucks to be L2 too? :)  Because in the end as long as
>> no one ever writes to spec_ctrl, everybody is happy.
> 
> Unfortunately, quite a few OS vendors shipped IBRS-based mitigations
> earlier this month. (Has Redhat stopped writing to IA32_SPEC_CTRL yet?
> :-)

Not yet, but getting there. :)

> And in the long run, everyone is going to set IA32_SPEC_CTRL.IBRS=1 on
> CPUs with IA32_ARCH_CAPABILITIES.IBRS_ALL.

And then it will suck for everyone---they will have to pay the price of
saving/restoring an MSR that is going to be written just once.  Perhaps
we will have to tweak the heuristic, only passing IBRS through when the
guest writes IBRS=0.

In the end I think it's premature to try and optimize for L2 guests of
long-lived L1 hypervisors.

Paolo