linux-kernel - Re: [PATCH v3 2/4] drivers/iommu: Add calls for IOMMU_DEBUG

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Message-ID: <aSV-Xi5uMJcMtk1b@google.com>
Date: Tue, 25 Nov 2025 10:01:02 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: linux-mm@...ck.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, corbet@....net, joro@...tes.org,
	will@...nel.org, robin.murphy@....com, akpm@...ux-foundation.org,
	vbabka@...e.cz, surenb@...gle.com, mhocko@...e.com,
	jackmanb@...gle.com, hannes@...xchg.org, ziy@...dia.com,
	david@...hat.com, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, rppt@...nel.org, xiaqinxin@...wei.com
Subject: Re: [PATCH v3 2/4] drivers/iommu: Add calls for IOMMU_DEBUG_PAGEALLOC

On Tue, Nov 25, 2025 at 03:35:08PM +0800, Baolu Lu wrote:
> On 11/25/25 04:08, Mostafa Saleh wrote:
> > Add calls for the new iommu debug config IOMMU_DEBUG_PAGEALLOC:
> > - iommu_debug_init: Enable the debug mode if configured by the user.
> > - iommu_debug_map: Track iommu pages mapped, using physical address.
> > - iommu_debug_unmap_begin: Track start of iommu unmap operation, with
> >    IOVA and size.
> > - iommu_debug_unmap_end: Track the end of unmap operation, passing the
> >    actual unmapped size versus the tracked one at unmap_begin.
> > 
> > We have to do the unmap_begin/end as once pages are unmapped we lose
> > the information of the physical address.
> > This is racy, but the API is racy by construction as it uses refcounts
> > and doesn't attempt to lock/synchronize with the IOMMU API as that will
> > be costly, meaning that possibility of false negative exists.
> > 
> > Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
> > ---
> >   drivers/iommu/iommu-debug-pagealloc.c | 26 +++++++++++++
> >   drivers/iommu/iommu.c                 | 12 +++++-
> >   include/linux/iommu-debug-pagealloc.h | 56 +++++++++++++++++++++++++++
> >   3 files changed, 92 insertions(+), 2 deletions(-)
> > 
> 
> Remove "drivers/" from the commit title.
> 
> $ git log --oneline drivers/iommu/iommu.c

My bad, I will fix it.

> 
> [...]
> > diff --git a/include/linux/iommu-debug-pagealloc.h b/include/linux/iommu-debug-pagealloc.h
> > index 83e64d70bf6c..454303ec09c2 100644
> > --- a/include/linux/iommu-debug-pagealloc.h
> > +++ b/include/linux/iommu-debug-pagealloc.h
> > @@ -8,10 +8,66 @@
> >   #ifndef __LINUX_IOMMU_DEBUG_PAGEALLOC_H
> >   #define __LINUX_IOMMU_DEBUG_PAGEALLOC_H
> > +struct iommu_domain;
> > +
> >   #ifdef CONFIG_IOMMU_DEBUG_PAGEALLOC
> > +DECLARE_STATIC_KEY_FALSE(iommu_debug_initialized);
> > +
> >   extern struct page_ext_operations page_iommu_debug_ops;
> > +void __iommu_debug_map(struct iommu_domain *domain, phys_addr_t phys,
> > +		       size_t size);
> > +void __iommu_debug_unmap_begin(struct iommu_domain *domain,
> > +			       unsigned long iova, size_t size);
> > +void __iommu_debug_unmap_end(struct iommu_domain *domain,
> > +			     unsigned long iova, size_t size, size_t unmapped);
> > +
> > +static inline void iommu_debug_map(struct iommu_domain *domain,
> > +				   phys_addr_t phys, size_t size)
> > +{
> > +	if (static_branch_unlikely(&iommu_debug_initialized))
> > +		__iommu_debug_map(domain, phys, size);
> > +}
> > +
> > +static inline void iommu_debug_unmap_begin(struct iommu_domain *domain,
> > +					   unsigned long iova, size_t size)
> > +{
> > +	if (static_branch_unlikely(&iommu_debug_initialized))
> > +		__iommu_debug_unmap_begin(domain, iova, size);
> > +}
> > +
> > +static inline void iommu_debug_unmap_end(struct iommu_domain *domain,
> > +					 unsigned long iova, size_t size,
> > +					 size_t unmapped)
> > +{
> > +	if (static_branch_unlikely(&iommu_debug_initialized))
> > +		__iommu_debug_unmap_end(domain, iova, size, unmapped);
> > +}
> > +
> > +void iommu_debug_init(void);
> > +
> > +#else
> > +static inline void iommu_debug_map(struct iommu_domain *domain,
> > +				   phys_addr_t phys, size_t size)
> > +{
> > +}
> > +
> > +static inline void iommu_debug_unmap_begin(struct iommu_domain *domain,
> > +					   unsigned long iova, size_t size)
> > +{
> > +}
> > +
> > +static inline void iommu_debug_unmap_end(struct iommu_domain *domain,
> > +					 unsigned long iova, size_t size,
> > +					 size_t unmapped)
> > +{
> > +}
> > +
> > +static inline void iommu_debug_init(void)
> > +{
> > +}
> 
> I suppose that all these should go to drivers/iommu/iommu-priv.h, as
> they are for use in other files inside the IOMMU subsystem.

It seemed better to have all the feature functions/declarations in one
isolated file, as it is included outside of the iommu susbsystem also.
I have no strong opinion, I can keep them in drivers/iommu/iommu-priv.h
if you think it's better. But then we will have to include also
"iommu-debug-pagealloc.h" for the static key to avoid including extra
files to linux/mm.h.

Thanks,
Mostafa

> 
> > +
> >   #endif /* CONFIG_IOMMU_DEBUG_PAGEALLOC */
> >   #endif /* __LINUX_IOMMU_DEBUG_PAGEALLOC_H */
> 
> Thanks,
> baolu