linux-kernel - Re: [PATCH v9 6/6] iommu/dma: Make flush queue sizes and timeout driver configurable

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Date:   Tue, 23 May 2023 14:20:24 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Will Deacon <will@...nel.org>,
        Wenjia Zhang <wenjia@...ux.ibm.com>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        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 v9 6/6] iommu/dma: Make flush queue sizes and timeout
 driver configurable

On Mon, 2023-05-22 at 18:10 +0200, Joerg Roedel wrote:
> On Mon, May 15, 2023 at 11:15:56AM +0200, Niklas Schnelle wrote:
> > In the s390 IOMMU driver a large fixed queue size and timeout is then
> > set together with single queue mode bringing its performance on s390
> > paged memory guests on par with the previous s390 specific DMA API
> > implementation.
> 
> Hmm, the right flush-queue size and timeout settings are more a function
> of the endpoint device and device driver than of the iommu driver, no? I
> think something like this could also help solving the recently reported
> scalability problems in the fq-code, if done right.
> 
> Regards,
> 
> 	Joerg
> 

In our case the large flush queue and timeout is needed because the
IOTLB flushes of the virtualized s390 IOMMU are used by KVM and z/VM to
synchronize their IOMMU shadow tables thus making them more expensive.
This then applies to all pass-through PCI devices without their drivers
knowing about the IOMMU being virtualized like that. But yes of course
there could be cases where the device driver knows better.

Thanks,
Niklas