Re: [RFC 0/8] KVM PCIe/MSI passthrough on ARM/ARM64 (Alt II)

[Alex Williamson]

On Thu,  3 Nov 2016 21:39:30 +0000
Eric Auger <eric.auger@xxxxxxxxxx> wrote:

> Following Will & Robin's suggestions, this series attempts to propose
> an alternative to [1] where the host would arbitrarily decide the
> location of the IOVA MSI window and would be able to report to the
> userspace the list of reserved IOVA regions that cannot be used
> along with VFIO_IOMMU_MAP_DMA. This would allow the userspace to react
> in case of conflict.
> 
> Userspace can retrieve all the reserved regions through the VFIO_IOMMU_GET_INFO
> IOCTL by querying the new RESV_IOVA_RANGE chained capability. Each reserved
> IOVA range is put in a separate capability.

Doesn't it make more sense to describe the non-holes (ie. what I
can use for DMA) rather the holes (what I can't use for DMA)?  For
example on VT-d, the IOMMU not only has the block of MSI addresses
handled through interrupt remapping, but it also has a maximum address
width.  Rather than describing the reserved space we could describe the
usable DMA ranges above and below that reserved block.

Anyway, there's also a pretty harsh problem that I came up with in
talking to Will.  If the platform describes a fixed IOVA range as
reserved, that's great for the use case when a VM is instantiated with
a device attached, but it seems like it nearly excludes the case of
hotplugging a device.  We can't dynamically decide that a set of RAM
pages in the VM cannot be used as a DMA target.  Does the user need to
create the VM with a predefined hole that lines up with the reserved
regions for this platform?  How do they know the reserved regions for
this platform?  How would we handle migration where an assigned device
hot-add might not occur until after we've migrated to a slightly
different platform from the one we started on, that might have
different reserved memory requirements?

We can always have QEMU reject hot-adding the device if the reserved
region overlaps existing guest RAM, but I don't even really see how we
advise users to give them a reasonable chance of avoiding that
possibility.  Apparently there are also ARM platforms where MSI pages
cannot be remapped to support the previous programmable user/VM
address, is it even worthwhile to support those platforms?  Does that
decision influence whether user programmable MSI reserved regions are
really a second class citizen to fixed reserved regions?  I expect
we'll be talking about this tomorrow morning, but I certainly haven't
come up with any viable solutions to this.  Thanks,

Alex

> At IOMMU level, the reserved regions are stored in an iommu_domain list
> which is populated on each device attachment. An IOMMU add_reserved_regions
> callback specializes the registration of the reserved regions.
> 
> On x86, the [FEE0_0000h - FEF0_000h] MSI window is registered (NOT tested).
> 
> On ARM, the PCI host bridge windows (ACS check to be added?) + the MSI IOVA
> reserved regions are populated by the arm-smmu driver. Currently the MSI
> IOVA region is arbitrarily located at 0x8000000 and 1MB sized.  An IOVA domain
> is created in add_reserved_regions callback. Then MSIs are transparently
> mapped using this IOVA domain.
> 
> This series currently does not address some features addressed in [1]:
> - MSI IOVA size requirement computation
> - IRQ safety assessment
> 
> This RFC was just tested on ARM Overdrive with QEMU and is sent to help
> potential discussions at LPC. Additionnal development + testing is needed.
> 
> 2 tentative fixes may be submitted separately:
> - vfio: fix vfio_info_cap_add/shift
> - iommu/iova: fix __alloc_and_insert_iova_range
> 
> Best Regards
> 
> Eric
> 
> [1] [PATCH v14 00/16] KVM PCIe/MSI passthrough on ARM/ARM64
> https://lkml.org/lkml/2016/10/12/347
> 
> Git: complete series available at
> https://github.com/eauger/linux/tree/v4.9-rc3-reserved-rfc
> 
> 
> Eric Auger (7):
>   vfio: fix vfio_info_cap_add/shift
>   iommu/iova: fix __alloc_and_insert_iova_range
>   iommu: Add a list of iommu_reserved_region in iommu_domain
>   vfio/type1: Introduce RESV_IOVA_RANGE capability
>   iommu: Handle the list of reserved regions
>   iommu/vt-d: Implement add_reserved_regions callback
>   iommu/arm-smmu: implement add_reserved_regions callback
> 
> Robin Murphy (1):
>   iommu/dma: Allow MSI-only cookies
> 
>  drivers/iommu/arm-smmu.c        | 63 +++++++++++++++++++++++++++++++++++++++++
>  drivers/iommu/dma-iommu.c       | 39 +++++++++++++++++++++++++
>  drivers/iommu/intel-iommu.c     | 48 ++++++++++++++++++++++---------
>  drivers/iommu/iommu.c           | 25 ++++++++++++++++
>  drivers/iommu/iova.c            |  2 +-
>  drivers/vfio/vfio.c             |  5 ++--
>  drivers/vfio/vfio_iommu_type1.c | 63 ++++++++++++++++++++++++++++++++++++++++-
>  include/linux/dma-iommu.h       |  9 ++++++
>  include/linux/iommu.h           | 23 +++++++++++++++
>  include/uapi/linux/vfio.h       | 16 ++++++++++-
>  10 files changed, 275 insertions(+), 18 deletions(-)
>