[v11,0/6] iommu/dma: s390 DMA API conversion and optimized IOTLB flushing

Message

Niklas Schnelle July 17, 2023, 11 a.m. UTC

Hi All,

This patch series converts s390's PCI support from its platform specific DMA
API implementation in arch/s390/pci/pci_dma.c to the common DMA IOMMU layer.
The conversion itself is done in patches 3-4 with patch 2 providing the final
necessary IOMMU driver improvement to handle s390's special IOTLB flush
out-of-resource indication in virtualized environments. Patches 1-2 may be
applied independently. The conversion itself only touches the s390 IOMMU driver
and s390 arch code moving over remaining functions from the s390 DMA API
implementation. No changes to common code are necessary.

After patch 3 the basic conversion is done and on our partitioning machine
hypervisor LPAR performance matches or exceeds the existing code. When running
under z/VM or KVM however, performance plummets to about half of the existing
code due to a much higher rate of IOTLB flushes for unmapped pages. Due to the
hypervisors use of IOTLB flushes to synchronize their shadow tables these are
very expensive and minimizing them is key for regaining the performance loss.

To this end patches 4-5 add a new, single queue, IOTLB flushing scheme as
an alternative to the existing per-CPU flush queues. Introducing an alternative
scheme was suggested by Robin Murphy[1]. In the previous RFC of this
conversion Robin suggested reusing more of the existing queuing logic which
I incorporated since v2. The single queue mode is introduced in patch
4 together with a new .shadow_on_flush flag bit in struct dev_iommu.
This allows IOMMU drivers to indicate that their IOTLB flushes do the
extra work of shadowing. This then lets the dma-iommu code use a single
queue.

Then patch 5 enables variable queue sizes using power of 2 values and
shift/mask to keep performance as close to the existing code as
possible. A larger queue size and timeout is then used by dma-iommu when
shadow_on_flush is set. This same scheme may also be used by other IOMMU
drivers with similar requirements. Particularly virtio-iommu may be
a candidate.

In a previous version I verified that the new scheme does work on my
x86_64 Ryzen workstation by locally modifying iommu_subsys_init() to
default to the single queue mode and verifying its use via
"/sys/.../iommu_group/type". I did not find problems with an AMD GPU,
Intel NIC (with SR-IOV and KVM pass-through), NVMes or any on board
peripherals.

This code is also available in the b4/dma_iommu topic branch of my
git.kernel.org repository[3] with tags the version sent.

NOTE: Due to the large drop in performance I think we should not merge
the DMA API conversion (patch 4) until we have a more suited IOVA
flushing scheme with similar improvements as the proposed changes.

Best regards,
Niklas

[0] https://lore.kernel.org/linux-iommu/[e-mail]/
[1] https://lore.kernel.org/linux-iommu/[e-mail]/
[2] https://lore.kernel.org/linux-iommu/[e-mail]/
[3] https://git.kernel.org/pub/scm/linux/kernel/git/niks/linux.git/

---
Changes in v11:
- Rebased on v6.5-rc2
- Added patch to force IOMMU_DOMAIN_DMA on s390 specific ISM devices
- Dropped the patch to properly set DMA mask on ISM devices which went upstream separately.
- s390 IOMMU driver now uses IOMMU_CAP_DEFERRED_FLUSH to enable DMA-FQ
  leaving no uses of IOMMU_DOMAIN_DMA_FQ in the driver.
- Link to v10: https://lore.kernel.org/r/[e-mail]

Changes in v10:
- Rebased on v6.4-rc3
- Removed the .tune_dma_iommu() op in favor of a .shadow_on_flush flag
  in struct dev_iommu which then let's the dma-iommu choose a single
  queue and larger timeouts and IOVA counts. This leaves the dma-iommu
  with full responsibility for the settings.
- The above change affects patches 5 and 6 and lead to a new subject for
  patch 6 since the flush queue size and timeout is no longer driver
  controlled
- Link to v9: https://lore.kernel.org/r/[e-mail]

Changes in v9:
- Rebased on v6.4-rc2
- Re-ordered iommu_group_store_type() to allow passing the device to
  iommu_dma_init_fq()
- Link to v8: https://lore.kernel.org/r/[e-mail]

---
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: Force ISM devices to use IOMMU_DOMAIN_DMA
      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

 Documentation/admin-guide/kernel-parameters.txt |   9 +-
 arch/s390/include/asm/pci.h                     |   7 -
 arch/s390/include/asm/pci_clp.h                 |   3 +
 arch/s390/include/asm/pci_dma.h                 | 119 +---
 arch/s390/pci/Makefile                          |   2 +-
 arch/s390/pci/pci.c                             |  22 +-
 arch/s390/pci/pci_bus.c                         |   5 -
 arch/s390/pci/pci_debug.c                       |  12 +-
 arch/s390/pci/pci_dma.c                         | 735 ------------------------
 arch/s390/pci/pci_event.c                       |  17 +-
 arch/s390/pci/pci_sysfs.c                       |  19 +-
 drivers/iommu/Kconfig                           |   4 +-
 drivers/iommu/amd/iommu.c                       |   5 +-
 drivers/iommu/apple-dart.c                      |   5 +-
 drivers/iommu/dma-iommu.c                       | 212 +++++--
 drivers/iommu/intel/iommu.c                     |   5 +-
 drivers/iommu/msm_iommu.c                       |   5 +-
 drivers/iommu/mtk_iommu.c                       |   5 +-
 drivers/iommu/s390-iommu.c                      | 433 ++++++++++++--
 drivers/iommu/sprd-iommu.c                      |   5 +-
 drivers/iommu/sun50i-iommu.c                    |   6 +-
 drivers/iommu/tegra-gart.c                      |   5 +-
 include/linux/iommu.h                           |   6 +-
 23 files changed, 641 insertions(+), 1005 deletions(-)
---
base-commit: fdf0eaf11452d72945af31804e2a1048ee1b574c
change-id: 20230310-dma_iommu-5e048c538647

Best regards,

Comments

Jason Gunthorpe Aug. 18, 2023, 4:51 p.m. UTC | #1

On Mon, Jul 17, 2023 at 01:00:00PM +0200, Niklas Schnelle wrote:
> Hi All,
> 
> This patch series converts s390's PCI support from its platform specific DMA
> API implementation in arch/s390/pci/pci_dma.c to the common DMA IOMMU layer.
> The conversion itself is done in patches 3-4 with patch 2 providing the final
> necessary IOMMU driver improvement to handle s390's special IOTLB flush
> out-of-resource indication in virtualized environments. Patches 1-2 may be
> applied independently. The conversion itself only touches the s390 IOMMU driver
> and s390 arch code moving over remaining functions from the s390 DMA API
> implementation. No changes to common code are necessary.

It has been another month with no comments, what is needed to get this
merged?

Getting the arches converted to the standard dma ops is a big deal,
can we please conclude this?

Thanks,
Jason