Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above slot validity checks

[Huang, Kai]

On 6/03/2024 1:38 pm, Sean Christopherson wrote:
> On Wed, Mar 06, 2024, Kai Huang wrote:
> > On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> > > Prioritize private vs. shared gfn attribute checks above slot validity
> > > checks to ensure a consistent userspace ABI.  E.g. as is, KVM will exit to
> > > userspace if there is no memslot, but emulate accesses to the APIC access
> > > page even if the attributes mismatch.
> >
> > IMHO, it would be helpful to explicitly say that, in the later case (emulate
> > APIC access page) we still want to report MEMORY_FAULT error first (so that
> > userspace can have chance to fixup, IIUC) instead of emulating directly,
> > which will unlikely work.
>
> Hmm, it's not so much that emulating directly won't work, it's that KVM would be
> violating its ABI.  Emulating APIC accesses after userspace converted the APIC
> gfn to private would still work (I think), but KVM's ABI is that emulated MMIO
> is shared-only.

But for (at least) TDX guest I recall we _CAN_ allow guest's MMIO to be 
mapped as private, right?  The guest is supposed to get a #VE anyway?

Perhaps I am missing something -- I apologize if this has already been 
discussed.

> FWIW, I doubt there's a legitmate use case for converting the APIC gfn to private,
> this is purely to ensure KVM has simple, consistent rules for how private vs.
> shared access work.

Again I _think_ for TDX APIC gfn can be private?  IIUC virtualizing APIC 
is done by the TDX module, which injects #VE to guest when emulation is 
required.