Re: [PATCH v2 1/1] iommu/sva: Invalidate KVA range on kernel TLB flush

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

On 7/15/25 20:25, Jason Gunthorpe wrote:
> On Tue, Jul 15, 2025 at 01:55:01PM +0800, Baolu Lu wrote:
>> Yes, the mm (struct mm of processes that are bound to devices) list is
>> an unbounded list and can theoretically grow indefinitely. This results
>> in an unpredictable critical region.
> 
> Every MM has a unique PASID so I don't see how you can avoid this.
>   
>> @@ -654,6 +656,9 @@ struct iommu_ops {
>>
>>   	int (*def_domain_type)(struct device *dev);
>>
>> +	void (*paging_cache_invalidate)(struct iommu_device *dev,
>> +					unsigned long start, unsigned long end);
> 
> How would you even implement this in a driver?
> 
> You either flush the whole iommu, in which case who needs a rage, or
> the driver has to iterate over the PASID list, in which case it
> doesn't really improve the situation.

The Intel iommu driver supports flushing all SVA PASIDs with a single
request in the invalidation queue. I am not sure if other IOMMU
implementations also support this, so you are right, it doesn't
generally improve the situation.

> 
> If this is a concern I think the better answer is to do a defered free
> like the mm can sometimes do where we thread the page tables onto a
> linked list, flush the CPU cache and push it all into a work which
> will do the iommu flush before actually freeing the memory.

Is it a workable solution to use schedule_work() to queue the KVA cache
invalidation as a work item in the system workqueue? By doing so, we
wouldn't need the spinlock to protect the list anymore.

Perhaps we would need another interface, perhaps named
iommu_sva_flush_kva_inv_wq(), to guarantee that all flush work is
completed before actually freeing the pages.

> One of the KPTI options might be easier at that point..
> 
> Jason

Thanks,
baolu