Re: [PATCH]0/2 Patches to furthure split kvm_init

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

Hollis Blanchard wrote:
> On Fri, 2007-11-30 at 18:03 +0800, Zhang, Xiantao wrote:
>   
>> Avi Kivity wrote:
>>     
>>> Zhang, Xiantao wrote:
>>>       
>>>>> Ah, I see.  It isn't just the alignment. How do you allocate
>>>>> kvm_vcpu, then? 
>>>>>
>>>>>           
>>>> For evevy vm, we allocate a big chunk of memory for structure
>>>> allocation. For vcpu, it should be always 64k aligned through our
>>>> allocation mechanism. So, we don't care about its aligment issue :)
>>>>
>>>>         
>>> I see.  Can you explain why you do that?  Do you have a special
>>> allocator in your guest-resident vmm module?
>>>       
>> Since our VMM module and KVM module will share the kvm and vcpu
>> structure, but VMM module has a different address space, so we have to
>> use fixed allocation method  to handle this share. For example, we
>> allocates 1M memory(1M align) for every vm for this purpose in kvm
>> module, and the first 64k is used for first vcpu of guest, and the
>> second 64 for the second vcpu, and same for other vcpus.  You can call
>> it as special allocator or other names:) This is determined by IA64
>> virtualization architecture, and hard to workaround it in this
>> host-based vm model. :(
>>     
>
> We're doing something similar with very large allocations.
>
> Currently, PowerPC's "vcpu" is actually a copy of the exception
> handlers, plus the real vcpu data structure at a higher offset. Since
> our exception handlers can't span 64KB regions, we allocate a full 64KB
> for each vcpu. I'm not sure what benefit a kmem_cache would have in this
> situation...
>
>   

A kmem_cache is useful for specifying alignment, and as a general
bookkeeping system.  It's nice to see how many objects you have
allocated in /proc/slabinfo, and it will automatically inform you if you
have a leak when you unload the module.

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


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