Re: [kvm-devel] [RFC/PATCH 01/15] preparation: provide hook to enable pgstes in user pagetable

[Avi Kivity <avi@qumranet.com>]

Martin Schwidefsky wrote:
> On Sun, 2008-03-23 at 12:15 +0200, Avi Kivity wrote:
>   
>>>> Can you convert the page tables at a later time without doing a
>>>> wholesale replacement of the mm?  It should be a bit easier to keep
>>>> people off the pagetables than keep their grubby mitts off the mm
>>>> itself.
>>>>     
>>>>         
>>> Yes, as far as I can see you're right. And whatever we do in arch code,
>>> after all it's just a work around to avoid a new clone flag.
>>> If something like clone() with CLONE_KVM would be useful for more
>>> architectures than just s390 then maybe we should try to get a flag.
>>>
>>> Oh... there are just two unused clone flag bits left. Looks like the
>>> namespace changes ate up a lot of them lately.
>>>
>>> Well, we could still play dirty tricks like setting a bit in current
>>> via whatever mechanism which indicates child-wants-extended-page-tables
>>> and then just fork and be happy.
>>>   
>>>       
>> How about taking mmap_sem for write and converting all page tables 
>> in-place?  I'd rather avoid the need to fork() when creating a VM.
>>     
>
> That was my initial approach as well. If all the page table allocations
> can be fullfilled the code is not too complicated. To handle allocation
> failures gets tricky. At this point I realized that dup_mmap already
> does what we want to do. It walks all the page tables, allocates new
> page tables and copies the ptes. In principle I would reinvent the wheel
> if we can not use dup_mmap

Well, dup_mm() can't work (and now that I think about it, for more 
reasons -- what if the process has threads?).

I don't think conversion is too bad.  You'd need a four-level loop to 
allocate and convert, and another loop to deallocate in case of error.  
If, as I don't doubt, s390 hardware can modify the ptes, you'd need 
cmpxchg to read and clear a pte in one operation.

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

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