Messages in this thread | | | Subject | Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest | From | Nadav Amit <> | Date | Wed, 10 Jan 2018 09:23:16 -0800 |
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Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 10/01/2018 18:14, Jim Mattson wrote: >>>> If (a) is true, does "IBRS ALL THE TIME" usage is basically a CPU >>>> change to just create all BTB/BHB entries to be tagged with >>>> prediction-mode at creation-time and that tag to be compared to current >>>> prediction-mode when CPU attempts to use BTB/BHB? >>> >>> I hope so, and I hope said prediction mode includes PCID/VPID too. >> >> Branch prediction entries should probably be tagged with PCID, VPID, >> EP4TA, and thread ID...the same things used to tag TLB contexts. > > But if so, I don't see the need for IBPB.
It is highly improbable that a microcode patch can change how prediction entries are tagged. IIRC, microcode may change the behavior of instructions and “assists" (e.g., TLB miss). Not much more than that.
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