Re: [PATCH v7 08/14] KVM: arm64: Enable KVM_CAP_MEMORY_FAULT_INFO and annotate fault in the stage-2 fault handler

[Oliver Upton <oliver.upton@linux.dev>]

On Mon, Mar 04, 2024 at 08:00:15PM +0000, Oliver Upton wrote:
> On Thu, Feb 15, 2024 at 11:53:59PM +0000, Anish Moorthy wrote:
> 
> [...]
> 
> > +	if (is_error_noslot_pfn(pfn)) {
> > +		kvm_prepare_memory_fault_exit(vcpu, gfn * PAGE_SIZE, PAGE_SIZE,
> > +					      write_fault, exec_fault, false);
> 
> Hmm... Reinterpreting the fault context into something that wants to be
> arch-neutral might make this a bit difficult for userspace to
> understand.
> 
> The CPU can take an instruction abort on an S1PTW due to missing write
> permissions, i.e. hardware cannot write to the stage-1 descriptor for an
> AF or DBM update. In this case HPFAR points to the IPA of the stage-1
> descriptor that took the fault, not the target page.
> 
> It would seem this gets expressed to userspace as an intent to write and
> execute on the stage-1 page tables, no?

Duh, kvm_vcpu_trap_is_exec_fault() (not to be confused with
kvm_vcpu_trap_is_iabt()) filters for S1PTW, so this *should*
shake out as a write fault on the stage-1 descriptor.

With that said, an architecture-neutral UAPI may not be able to capture
the nuance of a fault. This UAPI will become much more load-bearing in
the future, and the loss of granularity could become an issue.

Marc had some ideas about forwarding the register state to userspace
directly, which should be the right level of information for _any_ fault
taken to userspace.

-- 
Thanks,
Oliver