Re: [PATCH 1/2] iommu/debug: Add IOMMU page table dump debug facility

[Will Deacon <will@kernel.org>]

On Wed, Sep 10, 2025 at 10:58:09AM +0800, Qinxin Xia wrote:
> 
> 
> On 2025/9/9 21:06:33, Will Deacon <will@kernel.org> wrote:
> > On Thu, Aug 14, 2025 at 05:30:04PM +0800, Qinxin Xia wrote:
> > > +/**
> > > + * iova_info_dump - dump iova alloced
> > > + * @s - file structure used to generate serialized output
> > > + * @iovad: - iova domain in question.
> > > + */
> > > +static int iommu_iova_info_dump(struct seq_file *s, struct iommu_domain *domain)
> > > +{
> > > +	struct iova_domain *iovad;
> > > +	unsigned long long pfn;
> > > +	unsigned long i_shift;
> > > +	struct rb_node *node;
> > > +	unsigned long flags;
> > > +	size_t prot_size;
> > > +
> > > +	iovad = iommu_domain_to_iovad(domain);
> > > +	if (!iovad)
> > > +		return -ENOMEM;
> > > +
> > > +	i_shift = iova_shift(iovad);
> > > +
> > > +	/* Take the lock so that no other thread is manipulating the rbtree */
> > > +	spin_lock_irqsave(&iovad->iova_rbtree_lock, flags);
> > > +	assert_spin_locked(&iovad->iova_rbtree_lock);
> > > +
> > > +	for (node = rb_first(&iovad->rbroot); node; node = rb_next(node)) {
> > > +		struct iova *iova = rb_entry(node, struct iova, node);
> > > +
> > > +		if (iova->pfn_hi <= iova->pfn_lo)
> > > +			continue;
> > > +
> > > +		for (pfn = iova->pfn_lo; pfn <= iova->pfn_hi; ) {
> > > +			prot_size = domain->ops->dump_iova_prot(s, domain, pfn << i_shift);
> > > +			pfn = ((pfn << i_shift) + prot_size) >> i_shift;
> > > +		}
> > > +	}
> > > +
> > > +	spin_unlock_irqrestore(&iovad->iova_rbtree_lock, flags);
> > 
> > Why is the IOVA rbtree lock sufficient for serialising the page-table
> > accesses made by ->dump_iova_prot()? I don't see anything here that
> > prevents the walker walking into page-table pages that are e.g. being
> > freed or manipulated concurrently.
> > 
>  Thank you for catching this critical race condition.I will fix this in
>  next version. And,Jason suggests putting io_ptdump on top of iommu pt.
>  What do you think?

I guess it depends on whether or not you want to tie this debug feature
to drivers that support the new page-table stuff.

Will