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

[Paolo Bonzini]

On 18/02/21 17:15, Sean Christopherson wrote:
> 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;

Right you are, thanks.  Then the warning would catch this quick.  I 
queued 10 and 11 too, and will reply on 12 now that I looked at it more 
closely.

Paolo