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

[Baolu Lu]

On 12/13/25 02:44, Mostafa Saleh wrote:
> 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@xxxxxxxxxx>
> > > ---
> > >    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.

Okay, fair enough.

Thanks,
baolu