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

[Alexey Kardashevskiy]

On 07/11/17 19:42, Laurent Vivier wrote:
> 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?

I do not remember clearly but this kind of automation is usually less
manageable. What if we do not want huge IOMMU pages for some reason?


> 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?

Yes, TCE tables are still created in KVM so the size matters.

> Is it a way QEMU adds automatically the 0x1000000 mask to page_size_mask?

No. That thing which decides about -mem-path should also add the pgsz mask.


-- 
Alexey