Re: [PATCH v13 4/6] iommu/s390: Disable deferred flush for ISM devices

Re: [PATCH v13 4/6] iommu/s390: Disable deferred flush for ISM devices

[Matthew Rosato]

On 9/28/23 10:31 AM, 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).
> 
> 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 however 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 claiming IOMMU_CAP_DEFERRED_FLUSH only for non-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.
> 
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>

Looks like my review tag from v12 got dropped on accident -- no changes since prior version and still looks good to me so

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>

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Re: [PATCH v13 0/6] iommu/dma: s390 DMA API conversion and optimized IOTLB flushing

[Joerg Roedel]

On Thu, Sep 28, 2023 at 04:31:34PM +0200, Niklas Schnelle wrote:
> Niklas Schnelle (6):
>       iommu: Allow .iotlb_sync_map to fail and handle s390's -ENOMEM return
>       s390/pci: prepare is_passed_through() for dma-iommu
>       s390/pci: Use dma-iommu layer
>       iommu/s390: Disable deferred flush for ISM devices
>       iommu/dma: Allow a single FQ in addition to per-CPU FQs
>       iommu/dma: Use a large flush queue and timeout for shadow_on_flush

Applied, thanks Niklas.

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Re: [PATCH v13 0/6] iommu/dma: s390 DMA API conversion and optimized IOTLB flushing

[Niklas Schnelle]

On Mon, 2023-10-02 at 08:59 +0200, Joerg Roedel wrote:
> On Thu, Sep 28, 2023 at 04:31:34PM +0200, Niklas Schnelle wrote:
> > Niklas Schnelle (6):
> >       iommu: Allow .iotlb_sync_map to fail and handle s390's -ENOMEM return
> >       s390/pci: prepare is_passed_through() for dma-iommu
> >       s390/pci: Use dma-iommu layer
> >       iommu/s390: Disable deferred flush for ISM devices
> >       iommu/dma: Allow a single FQ in addition to per-CPU FQs
> >       iommu/dma: Use a large flush queue and timeout for shadow_on_flush
> 
> Applied, thanks Niklas.
> 

Thank you Joerg. During some more testing I've found a minor issue
where resetting the debug statistics still used the now obsolete
counters that should have been removed. So I'll send a follow up patch,
sorry for not noticing this sooner.

Thanks,
Niklas

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