netdev - Re: [PATCH v11 4/6] iommu/s390: Force ISM devices to use IOMMU_DOMAIN

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Date: Wed, 19 Jul 2023 08:56:55 -0400
From: Matthew Rosato <mjrosato@...ux.ibm.com>
To: Niklas Schnelle <schnelle@...ux.ibm.com>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>, Wenjia Zhang <wenjia@...ux.ibm.com>,
        Robin Murphy <robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: Gerd Bayer <gbayer@...ux.ibm.com>, Julian Ruess <julianr@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        Alexandra Winter
 <wintera@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev
 <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Hector Martin <marcan@...can.st>, Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Yong Wu <yong.wu@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Orson Zhai <orsonzhai@...il.com>,
        Baolin Wang
 <baolin.wang@...ux.alibaba.com>,
        Chunyan Zhang <zhang.lyra@...il.com>, Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Krishna Reddy
 <vdumpa@...dia.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Jonathan Corbet <corbet@....net>, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev, asahi@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-tegra@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v11 4/6] iommu/s390: Force ISM devices to use
 IOMMU_DOMAIN_DMA

On 7/17/23 7:00 AM, Niklas Schnelle wrote:
> ISM devices are virtual PCI devices used for cross-LPAR communication.
> Unlike real PCI devices ISM devices do not use the hardware IOMMU but
> inspects IOMMU translation tables directly on IOTLB flush (s390 RPCIT
> instruction).
> 
> While ISM devices keep their DMA allocations static and only very rarely
> DMA unmap at all, For each IOTLB flush that occurs after unmap the ISM
> devices will inspect the area of the IOVA space indicated by the flush.
> This means that for the global IOTLB flushes used by the flush queue
> mechanism the entire IOVA space would be inspected. In principle this
> would be fine, albeit potentially unnecessarily slow, it turns out
> however that ISM devices are sensitive to seeing IOVA addresses that are
> currently in use in the IOVA range being flushed. Seeing such in-use
> IOVA addresses will cause the ISM device to enter an error state and
> become unusable.
> 
> Fix this by forcing IOMMU_DOMAIN_DMA to be used for ISM devices. This
> makes sure IOTLB flushes only cover IOVAs that have been unmapped and
> also restricts the range of the IOTLB flush potentially reducing latency
> spikes.
> 
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>

This makes sense to me.

Reviewed-by: Matthew Rosato <mjrosato@...ux.ibm.com>