Re: [Qemu-devel] [PULL, 08/14] spapr_pci/spapr_pci_vfio: Support Dynamic DMA Windows (DDW)

[Laurent Vivier]

On 05/07/2016 07:31, David Gibson wrote:
> From: Alexey Kardashevskiy <address@hidden>
> 
> This adds support for Dynamic DMA Windows (DDW) option defined by
> the SPAPR specification which allows to have additional DMA window(s)
> 
> The "ddw" property is enabled by default on a PHB but for compatibility
> the pseries-2.6 machine and older disable it.
> This also creates a single DMA window for the older machines to
> maintain backward migration.
> 
> This implements DDW for PHB with emulated and VFIO devices. The host
> kernel support is required. The advertised IOMMU page sizes are 4K and
> 64K; 16M pages are supported but not advertised by default, in order to
> enable them, the user has to specify "pgsz" property for PHB and
> enable huge pages for RAM.

Why is it not advirtised by default?

When we start qemu with hugepage memory ("mount -t hugetlbfs none
/mnt/kvm_hugepage" and ".. -mem-path /mnt/kvm_hugepage .."), we have an
ugly message:

"qemu-kvm: System page size 0x1000000 is not enabled in page_size_mask
(0x11000). Performance may be slow"

I understand if we want to use this with VFIO, we need something like
"-global spapr-pci-host-bridge.pgsz=0x1011000".

But is it needed if we don't use VFIO?
Is it a way QEMU adds automatically the 0x1000000 mask to page_size_mask?

Thanks,
Laurent