vfio: powerpc/spapr: Support Dynamic DMA windows

[cascardo/linux.git] / Documentation / vfio.txt

diff --git a/Documentation/vfio.txt b/Documentation/vfio.txt
index dcc37e1..1dd3fdd 100644 (file)
--- a/Documentation/vfio.txt
+++ b/Documentation/vfio.txt
@@ -464,6 +464,25 @@ address is from pre-registered range.
 
 This separation helps in optimizing DMA for guests.
 
+6) sPAPR specification allows guests to have an additional DMA window(s) on
+a PCI bus with a variable page size. Two ioctls have been added to support
+this: VFIO_IOMMU_SPAPR_TCE_CREATE and VFIO_IOMMU_SPAPR_TCE_REMOVE.
+The platform has to support the functionality or error will be returned to
+the userspace. The existing hardware supports up to 2 DMA windows, one is
+2GB long, uses 4K pages and called "default 32bit window"; the other can
+be as big as entire RAM, use different page size, it is optional - guests
+create those in run-time if the guest driver supports 64bit DMA.
+
+VFIO_IOMMU_SPAPR_TCE_CREATE receives a page shift, a DMA window size and
+a number of TCE table levels (if a TCE table is going to be big enough and
+the kernel may not be able to allocate enough of physically contiguous memory).
+It creates a new window in the available slot and returns the bus address where
+the new window starts. Due to hardware limitation, the user space cannot choose
+the location of DMA windows.
+
+VFIO_IOMMU_SPAPR_TCE_REMOVE receives the bus start address of the window
+and removes it.
+
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 [1] VFIO was originally an acronym for "Virtual Function I/O" in its