Re: [PATCH v3 2/4] dma-iommu: Add iommu_dma_opt_mapping_size()

[John Garry]

On 08/06/2022 18:26, Bart Van Assche wrote:
> On 6/6/22 02:30, John Garry via iommu wrote:
> > +unsigned long iova_rcache_range(void)
> > +{
> > +    return PAGE_SIZE << (IOVA_RANGE_CACHE_MAX_SIZE - 1);
> > +}
>
> My understanding is that iova cache entries may be smaller than 
> IOVA_RANGE_CACHE_MAX_SIZE and hence that even if code that uses the DMA 
> mapping API respects this limit that a cache miss can still happen.

Sure, a cache miss may still happen - however once we have stressed the 
system for a while then the rcaches fill up and don't fail often, or 
often enough to be noticeable compared to not having a cached IOVAs at all.

Thanks,
john