Re: [PATCH v13 20/35] KVM: x86/mmu: Handle page fault for private memory

[Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx>]

On Sun, Nov 05, 2023 at 05:19:36PM +0100, Paolo Bonzini wrote:
> On Sun, Nov 5, 2023 at 2:04 PM Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx> wrote:
> >
> > > +static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> > > +                                           struct kvm_page_fault *fault)
> > > +{
> > > +     kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> > > +                                   PAGE_SIZE, fault->write, fault->exec,
> > > +                                   fault->is_private);
> > > +}
> > > +
> > > +static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> > > +                                struct kvm_page_fault *fault)
> > > +{
> > > +     int max_order, r;
> > > +
> > > +     if (!kvm_slot_can_be_private(fault->slot)) {
> > > +             kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > > +             return -EFAULT;
> > > +     }
> > > +
> > > +     r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
> > > +                          &max_order);
> > > +     if (r) {
> > > +             kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > > +             return r;
> >
> > Why report KVM_EXIT_MEMORY_FAULT here? even with a ret != -EFAULT?
> 
> The cases are EFAULT, EHWPOISON (which can report
> KVM_EXIT_MEMORY_FAULT) and ENOMEM. I think it's fine
> that even -ENOMEM can return KVM_EXIT_MEMORY_FAULT,
> and it doesn't violate the documentation.  The docs tell you "what
> can you do if error if EFAULT or EHWPOISON?"; they don't
> exclude that other errnos result in KVM_EXIT_MEMORY_FAULT,
> it's just that you're not supposed to look at it

Thanks, it's OK for ENOMEM + KVM_EXIT_MEMORY_FAULT.

Another concern is, now 3 places to report EFAULT + KVM_EXIT_MEMORY_FAULT:

  if (!kvm_slot_can_be_private(fault->slot)) {
	kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
	return -EFAULT;
  }

  file = kvm_gmem_get_file(slot);
  if (!file)
	return -EFAULT;

  if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
	kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
	return -EFAULT;
  }

They are different cases, and seems userspace should handle them
differently, but not enough information to distinguish them.

Thanks,
Yilun

> 
> Paolo
> 
>