Re: [PATCH] KVM: x86/mmu: Do not create SPTEs for GFNs that exceed host.MAXPHYADDR

[Sean Christopherson]

On Fri, Apr 29, 2022, Paolo Bonzini wrote:
> On 4/29/22 16:42, Sean Christopherson wrote:
> > On Fri, Apr 29, 2022, Paolo Bonzini wrote:
> > > On 4/29/22 16:24, Sean Christopherson wrote:
> > > > I don't love the divergent memslot behavior, but it's technically correct, so I
> > > > can't really argue.  Do we want to "officially" document the memslot behavior?
> > > > 
> > > 
> > > I don't know what you mean by officially document,
> > 
> > Something in kvm/api.rst under KVM_SET_USER_MEMORY_REGION.
> 
> Not sure if the API documentation is the best place because userspace does
> not know whether shadow paging is on (except indirectly through other
> capabilities, perhaps)?

Hrm, true, it's not like the userspace VMM can rewrite itself at runtime.

> It could even be programmatic, such as returning 52 for CPUID[0x80000008].
> A nested KVM on L1 would not be able to use the #PF(RSVD) trick to detect
> MMIO faults.  That's not a big price to pay, however I'm not sure it's a
> good idea in general...

Agreed, messing with CPUID is likely to end in tears.