[Lguest] [kvm-devel] [RFC PATCH 1/5] lguest: mmap backing file

[Avi Kivity]

Anthony Liguori wrote:
>>>     
>>>       
>> If we're going to mod the kernel, how about a "mmap this part of their address 
>> space" and having the kernel keep the mappings in sync.  But I think that if 
>> we want to get speed, we should probably be doing the copy between address 
>> spaces in-kernel so we can do lightweight exits.
>>   
>>     
>
> I don't think lightweight exits help the situation very much.  The 
> difference between a light weight and heavy weight exit is only 3-4k 
> cycles or so.
>   

On what host cpu?  IIRC the difference was bigger on Intel (and in 
relative terms, set to increase).

> in-kernel doesn't make the situation much easier.  You have to map pages 
> in from a different task.  It's a lot easier if you have both guest 
> mapped in userspace.
>   

The kernel already has everything mapped (kmap_atomic() is an addition 
on x86_64).



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