linux-kernel - Re: [PATCH v3 3/4] scsi: core: Cap shost max_sectors according to DMA optimum mapping limits

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Message-ID: <5b214e95-dd95-551a-496e-a2139a74e8eb@huawei.com>
Date:   Thu, 9 Jun 2022 09:00:50 +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-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <linux-scsi@...r.kernel.org>, <liyihang6@...ilicon.com>,
        <chenxiang66@...ilicon.com>, <thunder.leizhen@...wei.com>
Subject: Re: [PATCH v3 3/4] scsi: core: Cap shost max_sectors according to DMA
 optimum mapping limits

On 08/06/2022 22:07, Bart Van Assche wrote:
> On 6/8/22 10:50, John Garry wrote:
>> Please note that this limit only applies if we have an IOMMU enabled 
>> for the scsi host dma device. Otherwise we are limited by dma direct 
>> or swiotlb max mapping size, as before.
> 
> SCSI host bus adapters that support 64-bit DMA may support much larger 
> transfer sizes than 128 KiB.

Indeed, and that is my problem today, as my storage controller is 
generating DMA mapping lengths which exceeds 128K and they slow 
everything down.

If you say that SRP enjoys best peformance with larger transfers then 
can you please test this with an IOMMU enabled (iommu group type DMA or 
DMA-FQ)?

Thanks,
John