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

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 07, 2024, Kai Huang wrote:
> 
> 
> On 6/03/2024 3:02 pm, Sean Christopherson wrote:
> > On Wed, Mar 06, 2024, Kai Huang wrote:
> > > 
> > > 
> > > 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?
> > 
> > Not really.  KVM can't _map_ emulated MMIO as private memory, because S-EPT
> > entries can only point at convertible memory.
> 
> Right.  I was talking about the MMIO mapping in the guest, which can be
> private I suppose.
> 
> > KVM _could_ emulate in response to a !PRESENT EPT violation, but KVM is not
> > going to do that.
> > 
> > https://lore.kernel.org/all/ZcUO5sFEAIH68JIA@google.com
> > 
> 
> Right agreed KVM shouldn't "emulate" !PRESENT fault.

One clarification.  KVM *does* emulate !PRESENT faults.  And that's not optional,
as caching MMIO info in SPTEs is purely an optimization and isn't possible on all
CPUs, e.g. AMD CPUs with MAXPHYADDR=52 don't have reserved bits to set.

My point above was specifically about emulating *private* !PRESENT faults as MMIO.

> I am talking about this -- for TDX guest, if I recall correctly, for guest's
> MMIO gfn KVM still needs to get the EPT violation for the _first_ access, in
> which KVM can configure the EPT entry to make sure "suppress #VE" bit is
> cleared so the later accesses can trigger #VE directly.

That's totally fine, so long as the access is shared, not private.

> I suppose this is still the way we want to implement?
> 
> I am afraid for this case, we will still see the MMIO GFN as private, while
> by default I believe the "guest memory attributes" for that MMIO GFN should
> be shared?

No, the guest should know it's (emulated) MMIO and access the gfn as shared.  That
might generate a !PRESENT fault, but that's again a-ok.

> AFAICT, it seems the "guest memory attributes" code doesn't check whether a
> GFN is MMIO or truly RAM.

That's userspace's responsibility, not KVM's responsibility.  And if userspace
doesn't proactively make emulated MMIO regions shared, then the memory_fault exit
that results from this patch will give userspace the hook it needs to convert the
gfn to shared on-demand.