Re: Memory sharing among guest OSs

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Javier Guerra wrote:
> On 11/28/07, Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>   
>> Haifeng He wrote:
>>     
>>> I am pretty new to KVM. I have a question which is if it
>>> is possible (or easy) to share memory(such identical pages)
>>> among guest OSs in KVM?
>>>       
>> In KVM, guest memory is normal userspace memory.  So you can use any of
>> the standard Linux shared memory mechanisms to share memory among guests.
>>     
>
> so the question would be if QEMU has any mechanism to setup that
> shared memory, right?
>
>   

Right now, it doesn't.

> if so, how would that look to the guest OS?
>
>   

If done naively, part of guest memory would be shared among other
guests.  Unless the guest was modified to handle it, it would crash
fairly quickly.

One way to do it right is to define a "memory extension pci card" (say,
'qemu -memory-card /dev/shm/shared_mem -hda ...' to insert it into the
guest), and a guest PCI driver for it would map the memory into the
guest address space.  This should be fairly easy to do.

> i guess it would be nice if the "-net user" facility could emulate an
> RDMA-capable card using real shared memory, but AFAIK, RDMA doesn't
> look exactly like remote shared memory.
>
>   

No, RDMA is not transparent, while shared memory is.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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