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

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Fri, 2023-08-18 at 20:10 +0100, Robin Murphy wrote:
> 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.

Nice idea thank you. This is indeed less code, basically just return
zdev->pft != PCI_FUNC_TYPE_ISM for the IOMMU_CAP_DEFERRED_FLUSH check.
I think it's also semantically more clear in that we don't really care
about the domain type but about not getting deferred flushes.

Thanks,
Niklas

> 
> > Signed-off-by: Niklas Schnelle <schnelle@linux.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,
> > 
>