Re: [RFC PATCH part-5 00/22] VMX emulation

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 13, 2023, Jason Chen CJ wrote:
> This patch set is part-5 of this RFC patches. It introduces VMX
> emulation for pKVM on Intel platform.
> 
> Host VM wants the capability to run its guest, it needs VMX support.

No, the host VM only needs a way to request pKVM to run a VM.  If we go down the
rabbit hole of pKVM on x86, I think we should take the red pill[*] and go all the
way down said rabbit hole by heavily paravirtualizing the KVM=>pKVM interface.

Except for VMCALL vs. VMMCALL, it should be possible to eliminate all traces of
VMX and SVM from the interface.  That means no VMCS emulation, no EPT shadowing,
etc.  As a bonus, any paravirt stuff we do for pKVM x86 would also be usable for
KVM-on-KVM nested virtualization.

E.g. an idea floating around my head is to add a paravirt paging interface for
KVM-on-KVM so that L1's (KVM-high in this RFC) doesn't need to maintain its own
TDP page tables.  I haven't pursued that idea in any real capacity since most
nested virtualization use cases for KVM involve running an older L1 kernel and/or
a non-KVM L1 hypervisor, i.e. there's no concrete use case to justify the development
and maintenance cost.  But if the PV code is "needed" by pKVM anyways...

[*] You take the blue pill, the story ends, you wake up in your bed and believe
    whatever you want to believe. You take the red pill, you stay in wonderland,
    and I show you how deep the rabbit hole goes.

    -Morpheus