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

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Date:   Thu, 23 Jun 2022 09:36:12 +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 3/4] scsi: core: Cap shost max_sectors according to DMA
 optimum mapping limits

On 10/06/2022 16:37, John Garry via iommu wrote:
> 
>> On 6/9/22 10:54, John Garry wrote:
>>> ok, but do you have a system where the UFS host controller is behind 
>>> an IOMMU? I had the impression that UFS controllers would be mostly 
>>> found in embedded systems and IOMMUs are not as common on there.
>>
>> Modern phones have an IOMMU. Below one can find an example from a 
>> Pixel 6 phone. The UFS storage controller is not controller by the 
>> IOMMU as far as I can see but I wouldn't be surprised if the security 
>> team would ask us one day to enable the IOMMU for the UFS controller.
> 
> OK, then unfortunately it seems that you have no method to test. I might 
> be able to test USB MSC but I am not even sure if I can even get DMA 
> mappings who length exceeds the IOVA rcache limit there.

I was able to do some testing on USB MSC for an XHCI controller. The 
result is that limiting the max HW sectors there does not affect 
performance in normal conditions.

However if I hack the USB driver and fiddle with request queue settings 
then it can:
- lift max_sectors limit in usb_stor_host_template 120KB -> 256KB
- lift request queue read_ahead_kb 128KB -> 256KB

In this scenario I can get 42.5MB/s read throughput, as opposed to 
39.5MB/s in normal conditions. Since .can_queue=1 for that host it would 
not fall foul of some issues I experience in IOVA allocator performance, 
so limiting max_sectors would not be required for that reason.

So this is an artificial test, but it may be worth considering only 
applying this DMA mapping optimal max_sectors limit to SAS controllers 
which I know can benefit.

Christoph, any opinion?

thanks,
John