Jim Mattson <jmatt...@google.com> writes: > I'm not convinced that the "one size fits all" and "context-free" > approaches to VMCS shadowing are terribly effective. > > For example, we never shadow VMX_INSTRUCTION_INFO, but if we just > reflected an exit to L1 for which that field is defined, there's > probably a good chance that L1 will use it. We always shadow > VM_EXIT_INTR_INFO, but if we didn't just reflect exit reason 0 to L1, > it's not likely to be read. If the L2 guest is in legacy mode or > compatibility mode, L1 is much more likely to be interested in the > contents of the descriptor cache than if the guest is in 64-bit mode. > > Some hypervisors write TSC_OFFSET quite frequently. Others rarely. > Last time I checked (it's been a while), VirtualBox was always > interested in everything. :-) Kvm, Hyper-V, VMware, VirtualBox, > Parallels...they all have different patterns, and they change from > release to release. > > Is it worth having a set of VMCS shadowing bitmaps per-vCPU, in order > to make better use of this feature?
Per CPU or not, to improve the feature we'll probably need some sort of an 'adaptive' algorithm picking which fields to shadow. I haven't thought this through, especially read/write shadowing, but we can probably start with an empty bitmap and later shadow it when we get over some threshold of vmread/vmwrite exits we enabling shadowing. The question is when we un-shadow it. For example, we can un-shadow a field for writing every time we see it was not changed between two exits to L0 (so we're trying to write the same value to vmcs12). -- Vitaly
