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

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Date: Fri, 18 Aug 2023 20:10:15 +0100
From: Robin Murphy <robin.murphy@....com>
To: Niklas Schnelle <schnelle@...ux.ibm.com>, Joerg Roedel <joro@...tes.org>,
 Matthew Rosato <mjrosato@...ux.ibm.com>, Will Deacon <will@...nel.org>,
 Wenjia Zhang <wenjia@...ux.ibm.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 2023-07-17 12:00, 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.

Would it not be simpler to return false for IOMMU_CAP_DEFERRED_FLUSH for 
these devices?

Cheers,
Robin.

> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>   drivers/iommu/s390-iommu.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
> index f6d6c60e5634..020cc538e4c4 100644
> --- a/drivers/iommu/s390-iommu.c
> +++ b/drivers/iommu/s390-iommu.c
> @@ -710,6 +710,15 @@ struct zpci_iommu_ctrs *zpci_get_iommu_ctrs(struct zpci_dev *zdev)
>   	return &zdev->s390_domain->ctrs;
>   }
>   
> +static int s390_iommu_def_domain_type(struct device *dev)
> +{
> +	struct zpci_dev *zdev = to_zpci_dev(dev);
> +
> +	if (zdev->pft == PCI_FUNC_TYPE_ISM)
> +		return IOMMU_DOMAIN_DMA;
> +	return 0;
> +}
> +
>   int zpci_init_iommu(struct zpci_dev *zdev)
>   {
>   	u64 aperture_size;
> @@ -789,6 +798,7 @@ static const struct iommu_ops s390_iommu_ops = {
>   	.probe_device = s390_iommu_probe_device,
>   	.probe_finalize = s390_iommu_probe_finalize,
>   	.release_device = s390_iommu_release_device,
> +	.def_domain_type = s390_iommu_def_domain_type,
>   	.device_group = generic_device_group,
>   	.pgsize_bitmap = SZ_4K,
>   	.get_resv_regions = s390_iommu_get_resv_regions,
>