Re: [PATCH 0/7] KVM: Kernel-based Virtual Machine

[Avi Kivity <avi@qumranet.com>]

Arnd Bergmann wrote:
> I don't think it's that different. The Cell SPU scheduler is also
> implemented in kernel space. Every application using an SPU program
> has its own contexts in spufs and doesn't look at the others.
>
>   

Okay, I've misunderstood you before.


> While we don't have it yet, we're thinking about adding a sputop
> or something similar that shows the utilization of spus. You don't
> need that one, since get exactly that with the regular top, but you
> might want to have a tool that prints statistics about how often
> your guests drop out of the virtualisation mode, or the number
> of interrupts delivered to them.
>
>   

We have a debugfs interface and a kvm_stat script which shows exactly 
that (including a breakdown of the reasons for the exit).

>>> Have you thought about simply defining your guest to be a section
>>> of the processes virtual address space? That way you could use
>>> an anonymous mapping in the host as your guest address space, or
>>> even use a file backed mapping in order to make the state persistant
>>> over multiple runs. Or you could map the guest kernel into the
>>> guest real address space with a private mapping and share the
>>> text segment over multiple guests to save L2 and RAM.
>>>  
>>>       
>> I've thought of it but it can't work on i386 because guest physical
>> address space is larger than virtual address space on i386.  So we
>> mmap("/dev/kvm") with file offsets corresponding to guest physical
>> addresses.
>>
>> I still like that idea, since it allows using hugetlbfs and allowing
>> swapping.  Perhaps we'll just accept the limitation that guests on i386
>> are limited.
>>     
>
> What is the point of 32 bit hosts anyway? Isn't this only available
> on x86_64 type CPUs in the first place?
>   

No, 32-bit hosts are fully supported (except a 32-bit host can't run a 
32-bit guest).

Admittedly, virtualization is a memory-intensive operation, so a 64-bit 
host will usually be used.


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