Re: [PATCH v4 2/4] iommu: Add calls for IOMMU_DEBUG_PAGEALLOC

[Mostafa Saleh <smostafa@google.com>]

On Fri, Dec 12, 2025 at 10:33:20AM +0800, Baolu Lu wrote:
> On 12/11/25 20:59, 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@google.com>
> > ---
> >   drivers/iommu/iommu-debug-pagealloc.c | 28 +++++++++++++
> >   drivers/iommu/iommu-priv.h            | 58 +++++++++++++++++++++++++++
> >   drivers/iommu/iommu.c                 | 11 ++++-
> >   include/linux/iommu-debug-pagealloc.h |  1 +
> >   4 files changed, 96 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iommu/iommu-debug-pagealloc.c b/drivers/iommu/iommu-debug-pagealloc.c
> > index 4022e9af7f27..1d343421da98 100644
> > --- a/drivers/iommu/iommu-debug-pagealloc.c
> > +++ b/drivers/iommu/iommu-debug-pagealloc.c
> > @@ -5,11 +5,15 @@
> >    * IOMMU API debug page alloc sanitizer
> >    */
> >   #include <linux/atomic.h>
> > +#include <linux/iommu.h>
> >   #include <linux/iommu-debug-pagealloc.h>
> >   #include <linux/kernel.h>
> >   #include <linux/page_ext.h>
> > +#include "iommu-priv.h"
> > +
> >   static bool needed;
> > +DEFINE_STATIC_KEY_FALSE(iommu_debug_initialized);
> >   struct iommu_debug_metadata {
> >   	atomic_t ref;
> > @@ -25,6 +29,30 @@ struct page_ext_operations page_iommu_debug_ops = {
> >   	.need = need_iommu_debug,
> >   };
> > +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)
> > +{
> > +}
> > +
> > +void iommu_debug_init(void)
> > +{
> > +	if (!needed)
> > +		return;
> > +
> > +	pr_info("iommu: Debugging page allocations, expect overhead or disable iommu.debug_pagealloc");
> > +	static_branch_enable(&iommu_debug_initialized);
> > +}
> > +
> >   static int __init iommu_debug_pagealloc(char *str)
> >   {
> >   	return kstrtobool(str, &needed);
> > diff --git a/drivers/iommu/iommu-priv.h b/drivers/iommu/iommu-priv.h
> > index c95394cd03a7..aaffad5854fc 100644
> > --- a/drivers/iommu/iommu-priv.h
> > +++ b/drivers/iommu/iommu-priv.h
> > @@ -5,6 +5,7 @@
> >   #define __LINUX_IOMMU_PRIV_H
> >   #include <linux/iommu.h>
> > +#include <linux/iommu-debug-pagealloc.h>
> >   #include <linux/msi.h>
> >   static inline const struct iommu_ops *dev_iommu_ops(struct device *dev)
> > @@ -65,4 +66,61 @@ static inline int iommufd_sw_msi(struct iommu_domain *domain,
> >   int iommu_replace_device_pasid(struct iommu_domain *domain,
> >   			       struct device *dev, ioasid_t pasid,
> >   			       struct iommu_attach_handle *handle);
> > +
> > +#ifdef CONFIG_IOMMU_DEBUG_PAGEALLOC
> > +
> > +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);
> > +}
> 
> I am wondering whether it would be better if we move iommu_debug_map()
> to iommu-debug-pagealloc.c,
> 
> void iommu_debug_map(struct iommu_domain *domain,
> 		     phys_addr_t phys, size_t size)
> {
> 	if (static_branch_likely(&iommu_debug_initialized))
> 		__iommu_debug_map(domain, phys, size);
> }
> 
> (Does it make sense to use static_branch_likely() here? Normally, people
>  who enable CONFIG_IOMMU_DEBUG_PAGEALLOC would want to use this
>  debugging feature. Or not?)
> 
> So that ...

This actually was the v1 implementation [1], but Jörg suggested to move
it to a header file as a function call would have an overhead if this
feautre is disabled.

I believe the priority would be to keep the performance overhead minimal
with CONFIG_IOMMU_DEBUG_PAGEALLOC and the commandline disabled, so people
can run with the config in production and only enable the commandline
it to debug problems, without having overhead on the typical case.

> 
> > +
> > +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)
> > +{
> > +}
> > +
> > +#endif /* CONFIG_IOMMU_DEBUG_PAGEALLOC */
> > +
> >   #endif /* __LINUX_IOMMU_PRIV_H */
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 2ca990dfbb88..01b062575519 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -232,6 +232,8 @@ static int __init iommu_subsys_init(void)
> >   	if (!nb)
> >   		return -ENOMEM;
> > +	iommu_debug_init();
> > +
> >   	for (int i = 0; i < ARRAY_SIZE(iommu_buses); i++) {
> >   		nb[i].notifier_call = iommu_bus_notifier;
> >   		bus_register_notifier(iommu_buses[i], &nb[i]);
> > @@ -2562,10 +2564,12 @@ int iommu_map_nosync(struct iommu_domain *domain, unsigned long iova,
> >   	}
> >   	/* unroll mapping in case something went wrong */
> > -	if (ret)
> > +	if (ret) {
> >   		iommu_unmap(domain, orig_iova, orig_size - size);
> > -	else
> > +	} else {
> >   		trace_map(orig_iova, orig_paddr, orig_size);
> > +		iommu_debug_map(domain, orig_paddr, orig_size);
> > +	}
> >   	return ret;
> >   }
> > @@ -2627,6 +2631,8 @@ static size_t __iommu_unmap(struct iommu_domain *domain,
> >   	pr_debug("unmap this: iova 0x%lx size 0x%zx\n", iova, size);
> > +	iommu_debug_unmap_begin(domain, iova, size);
> > +
> >   	/*
> >   	 * Keep iterating until we either unmap 'size' bytes (or more)
> >   	 * or we hit an area that isn't mapped.
> > @@ -2647,6 +2653,7 @@ static size_t __iommu_unmap(struct iommu_domain *domain,
> >   	}
> >   	trace_unmap(orig_iova, size, unmapped);
> > +	iommu_debug_unmap_end(domain, orig_iova, size, unmapped);
> >   	return unmapped;
> >   }
> > diff --git a/include/linux/iommu-debug-pagealloc.h b/include/linux/iommu-debug-pagealloc.h
> > index 83e64d70bf6c..a439d6815ca1 100644
> > --- a/include/linux/iommu-debug-pagealloc.h
> > +++ b/include/linux/iommu-debug-pagealloc.h
> > @@ -9,6 +9,7 @@
> >   #define __LINUX_IOMMU_DEBUG_PAGEALLOC_H
> >   #ifdef CONFIG_IOMMU_DEBUG_PAGEALLOC
> > +DECLARE_STATIC_KEY_FALSE(iommu_debug_initialized);
> 
> ... we could make this static?
>

This is not static because of the header usage as mentioned above.

Thanks,
Mostafa

> >   extern struct page_ext_operations page_iommu_debug_ops;

[1] https://lore.kernel.org/linux-iommu/20251003173229.1533640-2-smostafa@google.com/
> 
> Thanks,
> baolu
>