Re: [Qemu-devel] [PATCH qemu] vfio/spapr: Allow backing bigger guest IOMMU pages with smaller physical pages

[Alexey Kardashevskiy]

On 3/5/18 11:03 am, David Gibson wrote:
> On Wed, May 02, 2018 at 06:59:53PM +1000, Alexey Kardashevskiy wrote:
>> On 2/5/18 4:37 pm, David Gibson wrote:
>>> On Wed, May 02, 2018 at 02:45:57PM +1000, Alexey Kardashevskiy wrote:
>>>> At the moment the PPC64/pseries guest only supports 4K/64K/16M IOMMU
>>>> pages and POWER8 CPU supports the exact same set of page size so
>>>> so far things worked fine.
>>>>
>>>> However POWER9 supports different set of sizes - 4K/64K/2M/1G and
>>>> the last two - 2M and 1G - are not even allowed in the paravirt interface
>>>> (RTAS DDW) so we always end up using 64K IOMMU pages, although we could
>>>> back guest's 16MB IOMMU pages with 2MB pages on the host.
>>>>
>>>> This stores the supported host IOMMU page sizes in VFIOContainer and uses
>>>> this later when creating a new DMA window.
>>>>
>>>> There should be no behavioral changes on platforms other than pseries.
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <address@hidden>
>>>
>>> What will happen if you try to use this on an older kernel without
>>> your mismatching pagesize changes?
>>
>> I tried 1GB huge pages and "-global spapr-pci-host-bridge.pgsz=0x1011000"
>> or 0x41011000.
> 
> With a hash guest or a radix guest?


The guest was radix but how would it change anything? The guest can only
ask for 4k/64k/16m for IOMMU.


-- 
Alexey