Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest

[Nadav Amit]

Paolo Bonzini <pbonzini@xxxxxxxxxx> 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 âassists" (e.g., TLB miss). Not much more than that.