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

[Baolu Lu <baolu.lu@linux.intel.com>]

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@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.

Okay, fair enough.

Thanks,
baolu