Re: kvm guest memory management

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

Dong, Eddie wrote:
>
>> But to the bigger picture.  We're quite close to using user-allocated
>> memory for the guest, instead of kernel allocated memory.  This means
>> that userspace will allocate a memory region and assign it to kvm as a
>> memory slot.  On x86-64, where we have a large address space,
>> this means
>> that all of memory can be in just one slot (well, slots also
>> allow us to
>> do tracking of dirty pages on a subset of memory, so maybe three slots
>> are needed).  In effect, the Linux process page tables become the g2h
>> (or p2m) tables, and access to guest memory is a simple
>> copy_to_user()/copy_from_user().
>>     
>
> There are couple reasons that g2h can't server this.
> A VT-d device or EPT/NPT table need to translate from guest physical 
> to machine physical address, while g2h uses host mode va as index.
> Other reason is that g2h also include host user space memory & kernel
> memory which guest should never touch. (A bad programmed VT-d device
> may modify the memory listed by VT-d table).
>
>   

For EPT, we can't use the host page tables because EPT does not support 
the dirty bit.  So EPT requires duplication of the page tables anyway.

> Dirty tracking can certainly be serviced even using p2m solely.
>
>   
>> User-allocated memory will enable the following features:
>> - s390 support
>> - guest swapping
>> - page migration (where a guest is migrated from one NUMA node
>> to another)
>> - in conjunction with a de-duplicating file system, page sharing
>> among guests 
>> - inter-guest shared memory (mmap() one file in two or more guests)
>> - easier use of huge pages
>> - more?
>>
>>     
>
> This doesn't conflict with my suggestion though the p2m table then
> need to be dynamically modifed in case swapping happens.
>   

The nice thing about using the host page tables is that it's 
automatically updated to reflect changes in mapping.  Translating a page 
(gfn_to_page) becomes a call to get_user_pages().


-- 
error compiling committee.c: too many arguments to function


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