Messages in this thread | | | Subject | Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL | From | Paolo Bonzini <> | 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
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