Re: Essay on an important Xen decision (long)

[Anthony Liguori <aliguori@us.ibm.com>]

Mark Williamson wrote:

>>I imagine that you would have to always have shadow paging enable but
>>you could still do bulk updates ala writable page tables so the
>>performance cost should be minimal I would think.
>>
>>Trying to understand the memory system in more details so any additional
>>info is much appreciate :-)
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>
>I don't see why that couldn't perform decently, although it'd have more 
>overhead than allowing the guest to manage its pagetables directly...  I 
>*thought* this was intended to be supported at some point, but I'm not sure 
>if it's been needed yet.  Others may have more concrete numbers for the 
>performance - I think writable PTs got benchmarked against shadowing at some 
>point.
>  
>
Just to be thorough, was the shadow paging code a "pure" shadow page 
table where ever PTE write trapped to the hypervisor or were bulk PMD 
updates sent to the hypervisor?

I'm surprised there would be a measurable difference with shadow paging 
as it should only require a potential allocation (which could be fast 
pathed) and in the normal case, a couple extra reads/writes.  I would 
think that cost would be overshadowed by the original cost of the 
context switch.

Of course, I guess it wouldn't be that much of a shock to me that the 
overhead is at least measurable...

Regards,

Anthony Liguori

>Cheers,
>Mark
>
>  
>
>>Thanks,
>>
>>Anthony Liguori
>>
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>>
>>>If ia64 does decide to back off from the P==M route then I suspect VP
>>>is the way to go (which is I think how ia64 domU's currently work
>>>anyway).
>>>
>>> -- Keir
>>>      
>>>
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>>    
>>
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