linux-kernel - Re: [PATCH v3 2/4] dma-iommu: Add iommu_dma_opt_mapping

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Date:   Wed, 8 Jun 2022 18:39:10 +0100
From:   John Garry <john.garry@...wei.com>
To:     Bart Van Assche <bvanassche@....org>,
        <damien.lemoal@...nsource.wdc.com>, <joro@...tes.org>,
        <will@...nel.org>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <hch@....de>,
        <m.szyprowski@...sung.com>, <robin.murphy@....com>
CC:     <linux-scsi@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <liyihang6@...ilicon.com>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v3 2/4] dma-iommu: Add iommu_dma_opt_mapping_size()

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