On Thu, Jun 18, 2015 at 09:37:22PM +1000, Alexey Kardashevskiy wrote:
(cut-n-paste from kernel patchset)
Each Partitionable Endpoint (IOMMU group) has an address range on a PCI bus
where devices are allowed to do DMA. These ranges are called DMA windows.
By default, there is a single DMA window, 1 or 2GB big, mapped at zero
on a PCI bus.
PAPR defines a DDW RTAS API which allows pseries guests
querying the hypervisor about DDW support and capabilities (page size mask
for now). A pseries guest may request an additional (to the default)
DMA windows using this RTAS API.
The existing pseries Linux guests request an additional window as big as
the guest RAM and map the entire guest window which effectively creates
direct mapping of the guest memory to a PCI bus.
This patchset reworks PPC64 IOMMU code and adds necessary structures
to support big windows.
Once a Linux guest discovers the presence of DDW, it does:
1. query hypervisor about number of available windows and page size masks;
2. create a window with the biggest possible page size (today 4K/64K/16M);
3. map the entire guest RAM via H_PUT_TCE* hypercalls;
4. switche dma_ops to direct_dma_ops on the selected PE.
Once this is done, H_PUT_TCE is not called anymore for 64bit devices and
the guest does not waste time on DMA map/unmap operations.
Note that 32bit devices won't use DDW and will keep using the default
DMA window so KVM optimizations will be required (to be posted later).
This patchset adds DDW support for pseries. The host kernel changes are
required, posted as:
[PATCH kernel v11 00/34] powerpc/iommu/vfio: Enable Dynamic DMA windows
This patchset is based on git://github.com/dgibson/qemu.git spapr-next branch.
A couple of general queries - this touchs on the kernel part as well
as the qemu part:
* Am I correct in thinking that the point in doing the
pre-registration stuff is to allow the kernel to handle PUT_TCE
in real mode? i.e. that the advatage of doing preregistration
rather than accounting on the DMA_MAP and DMA_UNMAP itself only
appears once you have kernel KVM+VFIO acceleration?