Re: [kvm-devel] mmu notifiers

[Avi Kivity <avi@qumranet.com>]

Robin Holt wrote:
> On Sat, Jan 12, 2008 at 09:51:56PM +0200, Avi Kivity wrote:
>   
>> Christoph Lameter wrote:
>>     
>>> On Thu, 10 Jan 2008, Avi Kivity wrote:
>>>
>>>   
>>>       
>>>> Actually sharing memory is possible even without this patch; one simply
>>>> mmap()s a file into the address space of both guests.  Or are you 
>>>> referring to
>>>> something else?
>>>>     
>>>>         
>>> A file from where? If a file is read by two guests then they will have 
>>> distinct page structs.
>>>
>>>   
>>>       
>> Two kvm instances mmap() the file (from anywhere) into the guest address 
>> space.  That memory is shared, and will be backed by the same page structs 
>> at the same offset.
>>     
>
> That sounds nice, but...
>
> For larger machine configurations, we have different memory access
> capabilities.  When a partition that is located close to the home node
> of the memory accesses memory, it is normal access.  When it is further
> away, they get special access to the line.  Before the shared line is
> sent to the reading node, it is converted by the memory controller into
> an exclusive request and the reading node is handed the only copy of
> the line.  If we gave a remote kernel access to the page, we would also
> open the entire owning nodes page tables up to speculative references
> which effectively would be viewed by hardware as cache-line contention.
>
> Additionally, we have needs beyond memory backed by files.  Including
> special devices which do not have struct pages at all (see mspec.c).
>   

I don't understand.

I was just explaining how kvm shares memory among guests (which does not 
require mmu notifiers); if you have some other configuration that can 
benefit from mmu notifiers, then, well, great.

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

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