Re: [PATCH] KVM: arm64: ptdump: Flush the last region

[Marc Zyngier]

On Mon, 20 Jul 2026 09:53:29 +0100,
Dev Jain <dev.jain@xxxxxxx> wrote:
> 
> 
> 
> On 18/07/26 4:42 am, Wei-Lin Chang wrote:
> > Currently the stage-2 ptdump calls note_page() at each leaf entry visit.
> > This simply misses the output of the last region, because note_page()
> > only dumps output when it detects a change in level/prot, or when the
> > walk enters a next marker section. The last region in the guest IPA
> > space with the same level/prot is not dumped since there is no change
> > after it.
> > 
> > To dump the final region, manually issue a note_page() call with address
> > BIT(ia_bits) (end of guest IPA space) and level == -1 to trigger a level
> > change after the last region. Additionally treat level == -1 as a last
> > flushing note_page() call and avoid dumping the name of the next marker
> > for this case.
> > 
> > Fixes: 7c4f73548ed1 ("KVM: arm64: Register ptdump with debugfs on guest creation")
> > Reported-by: Sashiko AI <sashiko-bot@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/kvmarm/20260630122758.891011F00A3A@xxxxxxxxxxxxxxx/
> > Signed-off-by: Wei-Lin Chang <weilin.chang@xxxxxxx>
> > ---
> >  arch/arm64/kvm/ptdump.c | 8 +++++---
> >  arch/arm64/mm/ptdump.c  | 5 +++--
> >  2 files changed, 8 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
> > index c9140e22abcf..4096e4a92fae 100644
> > --- a/arch/arm64/kvm/ptdump.c
> > +++ b/arch/arm64/kvm/ptdump.c
> > @@ -155,11 +155,13 @@ static int kvm_ptdump_guest_show(struct seq_file *m, void *unused)
> >  		.seq		= m,
> >  	};
> >  
> > -	write_lock(&kvm->mmu_lock);
> > +	guard(write_lock)(&kvm->mmu_lock);
> 
> The guard clause change has nothing to do with the fix I think? Although the change
> is small and the fix is for a recent commit so maybe we don't care having this
> in the same patch - not sure how strict kvm reviewers are about this : )
>

Look at the quality spaghetti code this would otherwise result in:

	write_lock();
	ret = kvm_pgtable_walk();
	if (ret) {
		write_unlock();
		return ret;
	}
	note_page_flush();
	write_unlock();

	return 0;

How is that better? So it has *everything* to do with the fix.

And FWIW, the policy for KVM is that we don't do lock->guard
conversions as a separate patches. Only when we need to change the
code, and as part of the patch that changes that code.  Which is
exactly what is happening here.

> 
> >  	ret = kvm_pgtable_walk(mmu->pgt, 0, BIT(mmu->pgt->ia_bits), &walker);
> > -	write_unlock(&kvm->mmu_lock);
> > +	if (ret)
> > +		return ret;
> > +	note_page(&st->parser_state.ptdump, BIT(mmu->pgt->ia_bits), -1, 0);
> >  
> > -	return ret;
> > +	return 0;
> >  }
> >  
> >  static int kvm_ptdump_guest_open(struct inode *m, struct file *file)
> > diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
> > index ab9899ca1e5f..fed4e4407e0e 100644
> > --- a/arch/arm64/mm/ptdump.c
> > +++ b/arch/arm64/mm/ptdump.c
> > @@ -194,6 +194,7 @@ void note_page(struct ptdump_state *pt_st, unsigned long addr, int level,
> >  	struct ptdump_pg_state *st = container_of(pt_st, struct ptdump_pg_state, ptdump);
> >  	struct ptdump_pg_level *pg_level = st->pg_level;
> >  	static const char units[] = "KMGTPE";
> > +	bool flush = level == -1;
> 
> Can we do something similar to what S1 ptdump does (note_page_flush in ptdump_walk_pgd).

See Mark's email.

	M.

-- 
Without deviation from the norm, progress is not possible.