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

[Robin Murphy]

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@xxxxxxxxxxxxx>
> ---
>   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,