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Message-ID: <2513a9d4-ac92-2e50-4861-bb65465cf1ae@linux.ibm.com>
Date: Fri, 29 Sep 2023 08:56:30 -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>,
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Vasily Gorbik <gor@...ux.ibm.com>,
Alexander Gordeev <agordeev@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Sven Schnelle <svens@...ux.ibm.com>,
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Sven Peter <sven@...npeter.dev>,
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Jonathan Hunter <jonathanh@...dia.com>,
Jonathan Corbet <corbet@....net>, linux-s390@...r.kernel.org,
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Subject: Re: [PATCH v13 4/6] iommu/s390: Disable deferred flush for ISM
devices
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@...ux.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@...ux.ibm.com>
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