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

[Will Deacon]

On Mon, May 18, 2026 at 02:45:59PM +0100, Leonardo Bras wrote:
> Hello Oliver, Will,
> Thanks for reviewing!
> 
> On Mon, May 18, 2026 at 09:52:16AM +0100, Will Deacon wrote:
> > On Mon, May 18, 2026 at 12:22:47AM -0700, Oliver Upton wrote:
> > > On Fri, May 15, 2026 at 08:59:02PM +0100, Leonardo Bras wrote:
> > > > Introduce S2 walker return values:
> > > > - SKIP_CHILDREN: skip walking the children of the current node
> > > > - SKIP_SIBLINGS: skip waling the siblings of the current node
> > > > 
> > > > Also, modify __kvm_pgtable_visit() to fulfil the hing on above return
> > > > values. Current walkers should not be impacted
> > > 
> > > I'd rather see something based around new walk flags than introducing an
> > > entirely new mechanic around return values.
> > > 
> > > e.g. you could split the LEAF flag into separate flags for blocks v.
> > > pages:
> > > 
> > > 	KVM_PGTABLE_WALK_PAGE,
> > > 	KVM_PGTABLE_WALK_BLOCK,
> > > 	KVM_PGTABLE_WALK_LEAF	= KVM_PGTABLE_WALK_PAGE |
> > > 				  KVM_PGTABLE_WALK_BLOCK,
> > > 
> > > and then let __kvm_pgtable_visit() decide how to steer the walk. You may
> > > need some special handling to get the address arithmetic right when
> > > skipping over a table of page descriptors.
> 
> I am probably not getting the whole inner workings of this solution, but 
> IIUC the idea would be to walk the blocks, but not the pages, right?
> 
> Blocks meaning level2- and pages being level3?
>  
> > 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?

Will