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Date:   Fri, 29 Sep 2023 08:56:30 -0400
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Subject: Re: [PATCH v13 4/6] iommu/s390: Disable deferred flush for ISM
 devices
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To:     Niklas Schnelle <schnelle@linux.ibm.com>,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Wenjia Zhang <wenjia@linux.ibm.com>,
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        Pierre Morel <pmorel@linux.ibm.com>,
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        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
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        Bjorn Andersson <andersson@kernel.org>,
        Konrad Dybcio <konrad.dybcio@linaro.org>,
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        <angelogioacchino.delregno@collabora.com>,
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        Thierry Reding <thierry.reding@gmail.com>,
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References: <20230928-dma_iommu-v13-0-9e5fc4dacc36@linux.ibm.com>
 <20230928-dma_iommu-v13-4-9e5fc4dacc36@linux.ibm.com>
From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <20230928-dma_iommu-v13-4-9e5fc4dacc36@linux.ibm.com>
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On 9/28/23 10:31 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).
> 
> 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 however 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 claiming IOMMU_CAP_DEFERRED_FLUSH only for non-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@linux.ibm.com>

Looks like my review tag from v12 got dropped on accident -- no changes since prior version and still looks good to me so

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