[RFC 0/1] tag IOMMU-protected devices in Dom0 fdt

[Roman Skakun <rm.skakun@xxxxxxxxx>]

From: Roman Skakun <roman_skakun@xxxxxxxx>

At the moment, Dom0 can't distinguish which devices are protected by
IOMMU and which are not. In some cases, this can cause swiotlb bounce
buffer use for DMA addresses above 32 bits, which in turn can lead
to poor performance. I started a conversation at [1], where we discussed
addition of a new device tree property to mark IOMMU-protected devices for Dom0.

As a result of negotiation at [1], I would like to present two
patches:

1. The first patch for the hypervisor. It adds a new device property
'xen,behind-iommu' to a relevant device node when the device is
IOMMU-protected.

2. The second patch is a Linux kernel counterpart. It detects the said
property and disables swiotlb for a device.

There is a possible issue: some devices may not be able to use DMA
addresses above 32 bit boundaries, so we can have problems in the
direct DMA mechanism when swiotlb-xen is disabled for a such
device. More generally, this can affect any device which DMA address
range is narrower than CPU one.
 
In this case,
the device DMA address should be no bigger than 32 bit boundaries for
each device that is not using swiotlb-xen.

Several ideas on how to overcome it:
1. Do not use high memory for Dom0.
2. Set DMA 32bit mask for each device if swiotlb is not used for this device.
3. Force balloon driver to allocate buffers only below 4GB.

I will be glad to get any comments or suggestions.

[1] 
https://lore.kernel.org/xen-devel/AM7PR03MB65936E5D0B25567D1B2FAECA85CC9@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

Roman Skakun (1):
  xen/arm: set iommu property for IOMMU-protected devices

 xen/arch/arm/domain_build.c | 7 +++++++
 1 file changed, 7 insertions(+)

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2.27.0