Re: [PATCH v3 17/22] KVM: Introduce KVM_CAP_ABSENT_MAPPING_FAULT without implementation

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 01, 2023, Oliver Upton wrote:
> Anish,
> 
> On Wed, Apr 12, 2023 at 09:35:05PM +0000, Anish Moorthy wrote:
> > +7.35 KVM_CAP_ABSENT_MAPPING_FAULT
> > +---------------------------------
> > +
> > +:Architectures: None
> > +:Returns: -EINVAL.
> > +
> > +The presence of this capability indicates that userspace may pass the
> > +KVM_MEM_ABSENT_MAPPING_FAULT flag to KVM_SET_USER_MEMORY_REGION to cause KVM_RUN
> > +to fail (-EFAULT) in response to page faults for which the userspace page tables
> > +do not contain present mappings. Attempting to enable the capability directly
> > +will fail.
> > +
> > +The range of guest physical memory causing the fault is advertised to userspace
> > +through KVM_CAP_MEMORY_FAULT_INFO (if it is enabled).
> 
> Maybe third time is the charm. I *really* do not like the
> interdependence between NOWAIT exits and the completely orthogonal
> annotation of existing EFAULT exits.

They're not completely orthogonal, because the touchpoints for NOWAIT are themselves
existing EFAULT exits.

> How do we support a userspace that only cares about NOWAIT exits but
> doesn't want other EFAULT exits to be annotated?

We don't.  The proposed approach is to not change the return value, and the
vcpu->run union currently holds random garbage on -EFAULT, so I don't see any reason
to require userspace to opt-in, or to let userspace opt-out.  I.e. fill
vcpu->run->memory_fault unconditionally (for the paths that are converted) and
advertise to userspace that vcpu->run->memory_fault *may* contain useful info on
-EFAULT when KVM_CAP_MEMORY_FAULT_INFO is supported.  And then we define KVM's
ABI such that vcpu->run->memory_fault is guarateed to be valid if an -EFAULT occurs
when faulting in guest memory (on supported architectures).

> It is very likely that userspace will only know how to resolve NOWAIT exits
> anyway. Since we do not provide a precise description of the conditions that
> caused an exit, there's no way for userspace to differentiate between NOWAIT
> exits and other exits it couldn't care less about.
> 
> NOWAIT exits w/o annotation (i.e. a 'bare' EFAULT) make even less sense
> since userspace cannot even tell what address needs fixing at that
> point.
> 
> This is why I had been suggesting we separate the two capabilities and
> make annotated exits an unconditional property of NOWAIT exits.

No, because as I've been stating ad nauseum, KVM cannot differentiate between a
NOWAIT -EFAULT and an -EFAULT that would have occurred regardless of the NOWAIT
behavior.  Defining the ABI to be that KVM fills memory_fault if and only if the
slot has NOWAIT will create a mess, e.g. if an -EFAULT occurs while userspace
is doing a KVM_SET_USER_MEMORY_REGION to set NOWAIT, userspace may or may not see
valid memory_fault information depending on when the vCPU grabbed its memslot
snapshot.