[PATCH v2 06/13] KVM: arm64: Add support for KVM_MEM_USERFAULT

[James Houghton]

On Wed, May 28, 2025 at 4:25 PM Sean Christopherson <seanjc at google.com> wrote:
>
> On Wed, May 28, 2025, James Houghton wrote:
> > On Wed, May 28, 2025 at 1:30 PM Sean Christopherson <seanjc at google.com> wrote:
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index c5d21bcfa3ed4..f1db3f7742b28 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -2127,15 +2131,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> >                                  const struct kvm_memory_slot *new,
> >                                  enum kvm_mr_change change)
> >  {
> > -     bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES;
> > +     u32 old_flags = old ? old->flags : 0;
> > +     u32 new_flags = new ? new->flags : 0;
> > +
> > +     /*
> > +      * If only changing flags, nothing to do if not toggling
> > +      * dirty logging.
> > +      */
> > +     if (change == KVM_MR_FLAGS_ONLY &&
> > +         !((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES))
> > +             return;
> >
> >       /*
> >        * At this point memslot has been committed and there is an
> >        * allocated dirty_bitmap[], dirty pages will be tracked while the
> >        * memory slot is write protected.
> >        */
> > -     if (log_dirty_pages) {
> > -
> > +     if (new_flags & KVM_MEM_LOG_DIRTY_PAGES) {
> >               if (change == KVM_MR_DELETE)
> >                       return;
> >
> >
> > So we need to bail out early if we are enabling KVM_MEM_USERFAULT but
> > KVM_MEM_LOG_DIRTY_PAGES is already enabled, otherwise we'll be
> > write-protecting a bunch of PTEs that we don't need or want to WP.
> >
> > When *disabling* KVM_MEM_USERFAULT, we definitely don't want to WP
> > things, as we aren't going to get the unmap afterwards anyway.
> >
> > So the check we started with handles this:
> > > > > > +       u32 old_flags = old ? old->flags : 0;
> > > > > > +       u32 new_flags = new ? new->flags : 0;
> > > > > > +
> > > > > > +       /* Nothing to do if not toggling dirty logging. */
> > > > > > +       if (!((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES))
> > > > > > +               return;
> >
> > So why also check for `change == KVM_MR_FLAGS_ONLY` as well? Everything I just
> > said doesn't really apply when the memslot is being created, moved, or
> > destroyed. Otherwise, consider the case where we never enable dirty logging:
> >
> >  - Memslot deletion would be totally broken; we'll see that
> >    KVM_MEM_LOG_DIRTY_PAGES is not getting toggled and then bail out, skipping
> >    some freeing.
>
> No, because @new and thus new_flags will be 0.  If dirty logging wasn't enabled,
> then there's nothing to be done.
>
> >  - Memslot creation would be broken in a similar way; we'll skip a bunch of
> >    setup work.
>
> No, because @old and thus old_flags will be 0.  If dirty logging isn't being
> enabled, then there's nothing to be done.
>
> >  - For memslot moving, the only case that we could possibly be leaving
> >    KVM_MEM_LOG_DIRTY_PAGES set without the change being KVM_MR_FLAGS_ONLY,
> >    I think we still need to do the split and WP stuff.
>
> No, because KVM invokes kvm_arch_flush_shadow_memslot() on the memslot and marks
> it invalid prior to installing the new, moved memslot.  See kvm_invalidate_memslot().
>
> So I'm still not seeing what's buggy.

Sorry, I didn't see your reply, Sean. :(

You're right, I was confusing the KVM_MEM_USERFAULT and
KVM_MEM_LOG_DIRTY_PAGES. I'll undo the little change I said I was
going to make.

Thank you!