Re: [PATCH 10/14] KVM: x86: Further clarify the logic and comments for toggling log dirty

Sean Christopherson
2021-02-18T18:27:37Z

On Thu, Feb 18, 2021, Paolo Bonzini wrote:
> On 13/02/21 01:50, Sean Christopherson wrote:
> >
> > - * Nothing to do for RO slots or CREATE/MOVE/DELETE of a slot.
> > - * See comments below.
> > + * Nothing to do for RO slots (which can't be dirtied and can't be made
> > + * writable) or CREATE/MOVE/DELETE of a slot. See comments below.
> > */
> > if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
> > return;
> > + /*
> > + * READONLY and non-flags changes were filtered out above, and the only
> > + * other flag is LOG_DIRTY_PAGES, i.e. something is wrong if dirty
> > + * logging isn't being toggled on or off.
> > + */
> > + if (WARN_ON_ONCE(!((old->flags ^ new->flags) & KVM_MEM_LOG_DIRTY_PAGES)))
> > + return;
> > +
>
> What about readonly -> readwrite changes?

Not allowed without first deleting the memslot. See commit 75d61fbcf563 ("KVM:
set_memory_region: Disallow changing read-only attribute later"). RW->RO is
also not supported.

if (!old.npages) {
change = KVM_MR_CREATE;
new.dirty_bitmap = NULL;
memset(&new.arch, 0, sizeof(new.arch));
} else { /* Modify an existing slot. */
if ((new.userspace_addr != old.userspace_addr) ||
(new.npages != old.npages) ||
((new.flags ^ old.flags) & KVM_MEM_READONLY))
return -EINVAL;