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Message-ID: <ba1e0b29-52e0-2fc0-2eb9-475735febacf@arm.com>
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>,
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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>,
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Sven Schnelle <svens@...ux.ibm.com>,
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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,
>
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