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Date:   Fri, 8 Jul 2022 17:17:06 +0100
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Subject: Re: [PATCH v5 0/5] DMA mapping changes for SCSI core
To:     "Martin K. Petersen" <martin.petersen@oracle.com>,
        Christoph Hellwig <hch@lst.de>
CC:     <damien.lemoal@opensource.wdc.com>, <joro@8bytes.org>,
        <will@kernel.org>, <jejb@linux.ibm.com>,
        <m.szyprowski@samsung.com>, <robin.murphy@arm.com>,
        <linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-ide@vger.kernel.org>, <iommu@lists.linux-foundation.org>,
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        <linuxarm@huawei.com>
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From:   John Garry <john.garry@huawei.com>
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On 07/07/2022 21:35, Martin K. Petersen wrote:
> Christoph,
> 
>> Yes, I've mostly been waiting for an ACK from Martin.
> Sorry, I'm on vacation this week. The series looks OK to me although I
> do agree that it would be great if the max was reflected in the queue's
> hard limit and opt in the soft limit.

Ah, I think that I misunderstood Damien's question. I thought he was 
asking why not keep shost max_sectors at dma_max_mapping_size() and then 
init each sdev request queue max hw sectors at dma_opt_mapping_size().

But he seems that you want to know why not have the request queue max 
sectors at dma_opt_mapping_size(). The answer is related to meaning of 
dma_opt_mapping_size(). If we get any mappings which exceed this size 
then it can have a big dma mapping performance hit. So I set max hw 
sectors at this ‘opt’ mapping size to ensure that we get no mappings 
which exceed this size. Indeed, I think max sectors is 128Kb today for 
my host, which would be same as dma_opt_mapping_size() value with an 
IOMMU enabled. And I find that only a small % of request size may exceed 
this 128kb size, but it still has a big performance impact.

> 
> Acked-by: Martin K. Petersen<martin.petersen@oracle.com>

Thanks,
John