Re: [PATCH v3 2/3] KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags

[Leonardo Bras <leo.bras@arm.com>]

On Wed, Jul 08, 2026 at 04:49:01PM +0100, Leonardo Bras wrote:
> On Wed, Jul 08, 2026 at 01:50:11PM +0000, sashiko-bot@kernel.org wrote:
> > Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> > - [Medium] KVM_PGTABLE_WALK_SKIP_LEVEL* flags fail to skip the starting root level of the page table walk.
> > --
> > 
> > --- Patch [2]: [PATCH v3 2/3] KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags ---
> > commit 5ddf63ba492be8c2a84af0b81cf00790fc72767a
> > Author: Leonardo Bras <leo.bras@arm.com>
> > 
> > KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags
> > 
> > This commit adds new walking flags that tell kvm_pgtable_walk() to skip
> > lower levels when traversing page tables.
> > 
> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > index 4be1d51a6ac58..b9a2078efc519 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > 
> > [ ... ]
> > 
> > > @@ -192,7 +205,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
> > >  	if (!kvm_pgtable_walk_continue(data->walker, ret))
> > >  		return ret;
> > >  
> > > -	if (!table) {
> > > +	if (!table || kvm_pgtable_skip_level(level + 1, ctx.flags)) {
> > 
> > [Severity: Medium]
> > Does this fail to skip the starting level of the page table walk if that
> > initial level matches the skip criteria?
> > 
> > Looking at __kvm_pgtable_visit() in arch/arm64/kvm/hyp/pgtable.c, the visitor
> > callbacks are executed unconditionally for the current level before we reach
> > this skip check:
> > 
> > 	if (table && (ctx.flags & KVM_PGTABLE_WALK_TABLE_PRE)) {
> > 		ret = kvm_pgtable_visitor_cb(data, &ctx, KVM_PGTABLE_WALK_TABLE_PRE);
> > 		reload = true;
> > 	}
> > 
> > 	if (!table && (ctx.flags & KVM_PGTABLE_WALK_LEAF)) {
> > 		ret = kvm_pgtable_visitor_cb(data, &ctx, KVM_PGTABLE_WALK_LEAF);
> > 		reload = true;
> > 	}
> > 
> > Since the skip condition here only evaluates level + 1 to prevent descending
> > further, if a caller begins a walk at a level that should be skipped (for
> > example, starting at level 2 with KVM_PGTABLE_WALK_SKIP_LEVEL2), the callbacks
> > for that root level will still execute. 
> > 
> > Should the current level also be checked against the skip flags before
> > executing the pre-table or leaf visitor callbacks?
> 
> Yes, we do execute the callback at least in the root we are given, which 
> can have bad results in some scenario. That being said, testing here is not 
> the best place, as this function will be called a lot.
> 
> If we want to avoid that scenario, I suggest we do that test before 
> starting the walk, which would be on  _kvm_pgtable_walk(), except that both
> kvm_pgtable_stage2_create_unlinked(), and 
> kvm_pgtable_stage2_free_unlinked()
> will call _kvm_pgtable_walk() directly.

will call __kvm_pgtable_walk() directly.

> 
> (although they don't use skip_level paramters, yet)
> 
> Will take a look on that.
> 
> Thanks!
> Leo