[RFC PATCH 1/2] KVM: arm64: Introduce S2 walker SKIP return options

[Will Deacon]

On Tue, May 19, 2026 at 01:56:48PM +0100, Leonardo Bras wrote:
> On Tue, May 19, 2026 at 01:43:37PM +0100, Will Deacon wrote:
> > > > I was wondering along similar lines, but maybe it would be useful just
> > > > to pass a maximum level to the walker logic? That feels like the most
> > > > general case without complicating the existing logic.
> > > 
> > > This proposal seems simpler for me to understand, and indeed looks like a 
> > > better solution than what I have proposed, taking care of  the 
> > > 'already split' case with better performance, as it don't even walk a 
> > > single level-3 entry. 
> > > 
> > > On the 'splitting' case, it also works flawlessly if the memory is given in 
> > > level-2 blocks. There is only one case that I would like to address here:
> > > 
> > > - Memory given in level-1 blocks (say 1GB)
> > > - Walker flag says 'walk down to level-2 only'
> > > - Split Walker on level-1 will break page down to (up to) level-3 entries.
> > > - Walker will continue to be called on level-2 entries, even though it's 
> > >   not necessary.
> > 
> > If you're only visiting leaves, why would it be called on the level-2
> > table entries?
> > 
> 
> Because once the leaf is turned into a table by the splitting walker, it 
> gets reloaded and walked. This is an excerpt of __kvm_pgtable_visit():

Sorry, I was musing about the semantics after adding something to limit
the maximum level. I don't dispute what the current code would do.

> Example:
> - Split this level-1 leave:
>   - Walker creates the whole structure up to given level (currently 3)
>   - Walker returns, gets reloaded, table detected, go down on that one
>   - Level 2 entries walked (which is unnecessary)
> 
> Please let me know if I am misunderstanding something.

I just don't grok why this would happen if we limited the maximum level
to '2' _and_ said we only wanted to visit the leaf entries. In that
case, I wouldn't expect to descend into any of the L2 table entries
(because that would imply going beyond level 2) and I wouldn't expect to
be called for the table entries either (because we're only interested in
leaves).

Will