Re: [PATCH v2 2/4] drivers/iommu: Add calls for IOMMU_DEBUG_PAGEALLOC

[Mostafa Saleh <smostafa@google.com>]

On Thu, Nov 13, 2025 at 11:00:15AM +0000, Will Deacon wrote:
> On Thu, Nov 06, 2025 at 04:39:51PM +0000, 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: Track iommu pages unmapped, using IO virtual
> >   address.
> > - iommu_debug_remap: Track iommu pages, already mapped using IOVA.
> > 
> > We have to do the unmap/remap 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>
> > Tested-by: Qinxin Xia <xiaqinxin@huawei.com>
> > ---
> >  drivers/iommu/iommu-debug-pagealloc.c | 23 ++++++++++++
> >  drivers/iommu/iommu.c                 | 14 ++++++-
> >  include/linux/iommu-debug-pagealloc.h | 54 +++++++++++++++++++++++++++
> >  3 files changed, 89 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iommu/iommu-debug-pagealloc.c b/drivers/iommu/iommu-debug-pagealloc.c
> > index 385c8bfae02b..a6a2f844b09d 100644
> > --- a/drivers/iommu/iommu-debug-pagealloc.c
> > +++ b/drivers/iommu/iommu-debug-pagealloc.c
> > @@ -5,11 +5,13 @@
> >   * 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>
> >  
> >  static bool needed;
> > +DEFINE_STATIC_KEY_FALSE(iommu_debug_initialized);
> >  
> >  struct iommu_debug_metadate {
> >  	atomic_t ref;
> > @@ -25,6 +27,27 @@ 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(struct iommu_domain *domain, unsigned long iova, size_t size)
> > +{
> > +}
> > +
> > +void __iommu_debug_remap(struct iommu_domain *domain, unsigned long iova, size_t size)
> > +{
> > +}
> 
> Since the IOMMU API doesn't really have a "remap" operation, I wonder
> whether it would be clearer to have unmap_begin() and unmap_end()
> functions instead? You'd probably want to call them as a pair, so the
> check for unmapped < size would move into unmap_end().

I guess that would be cleaner, but we have to pass both size and
unmapped to unmap_end, I will give it a try.

Thanks,
Mostafa

> 
> Will