Re: [PATCH v3 04/15] KVM: Add common infrastructure for KVM Userfaults

[James Houghton <jthoughton@google.com>]

On Wed, Jun 18, 2025 at 12:41 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> On Wed, Jun 18, 2025 at 04:24:13AM +0000, James Houghton wrote:
> > KVM Userfault consists of a bitmap in userspace that describes which
> > pages the user wants exits on (when KVM_MEM_USERFAULT is enabled). To
> > get those exits, the memslot where KVM_MEM_USERFAULT is being enabled
> > must drop (at least) all of the translations that the bitmap says should
> > generate faults. Today, simply drop all translations for the memslot. Do
> > so with a new arch interface, kvm_arch_userfault_enabled(), which can be
> > specialized in the future by any architecture for which optimizations
> > make sense.
> >
> > Make some changes to kvm_set_memory_region() to support setting
> > KVM_MEM_USERFAULT on KVM_MEM_GUEST_MEMFD memslots, including relaxing
> > the retrictions on guest_memfd memslots from only deletion to no moving.
> >
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
>
> > +#ifdef CONFIG_KVM_GENERIC_PAGE_FAULT
> > +bool kvm_do_userfault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>
> The polarity of the return here feels weird. If we want a value of 0 to
> indicate success then int is a better return type.

The way it's written now feels fine to me. I'm happy to change it to
an int (where we return -EFAULT instead of 'true' and 0 instead of
'false').

> > +{
> > +     struct kvm_memory_slot *slot = fault->slot;
> > +     unsigned long __user *user_chunk;
> > +     unsigned long chunk;
> > +     gfn_t offset;
> > +
> > +     if (!kvm_is_userfault_memslot(slot))
> > +             return false;
> > +
> > +     offset = fault->gfn - slot->base_gfn;
> > +     user_chunk = slot->userfault_bitmap + (offset / BITS_PER_LONG);
> > +
> > +     if (__get_user(chunk, user_chunk))
> > +             return true;
> > +
>
> I see that the documentation suggests userspace perform a store-release
> to update the bitmap. That's the right idea but we need a load-acquire
> on the consumer side for that to do something meaningful.

Indeed, the below test_bit() should be test_bit_acquire(), thank you!

(N.B. I don't think the current code could result in an observable
bug, given that the later write of the PTE has a control dependency
here. But it is certainly written incorrectly.)

> > +     if (!test_bit(offset % BITS_PER_LONG, &chunk))
> > +             return false;
> > +
> > +     kvm_prepare_memory_fault_exit(vcpu, fault);
> > +     vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_USERFAULT;
> > +     return true;
> > +}
> > +#endif
> > +
> >  int __attribute__((weak)) kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >                                                 struct kvm_enable_cap *cap)
> >  {
> > --
> > 2.50.0.rc2.692.g299adb8693-goog
> >
>
> Thanks,
> Oliver