Re: [Qemu-devel] Re: [PATCH v2] qemu-kvm: Speed up of the dirty-bitmap-traveling

[Alexander Graf]

Avi Kivity wrote:
> On 02/10/2010 06:47 PM, Alexander Graf wrote:
>   
>>>> Because on PPC, you usually run PPC32 userspace code on a PPC64 kernel.
>>>> Unlike with x86, there's no real benefit in using 64 bit userspace.
>>>>   
>>>>     
>>>>       
>>>>         
>>> btw, does 32-bit ppc qemu support large memory guests? It doesn't on
>>> x86, and I don't remember any hacks to support large memory guests
>>> elsewhere.
>>>   
>>>     
>>>       
>> It doesn't :-). In fact, the guest we virtualize wouldn't work with > 2
>> GB anyways, because that needs an iommu implementation.
>>   
>>     
>
> Oh, so you may want to revisit the "there's no real benefit in using 64
> bit userspace".
>   

Well, for normal users they don't. SLES11 is 64-bit only, so we're good
on that. But openSUSE uses 32-bit userland.

> Seriously, that looks like a big deficiency. What would it take to
> implement an iommu?
>
> I imagine Anthony's latest patches are a first step in that journey.
>   

All reads/writes from PCI devices would need to go through a wrapper.
Maybe we could also define a per-device offset for memory accesses. That
way the overhead might be less.

Yes, Anthony's patches look like they are a really big step in that
direction.


Alex