Re: [PATCH 1/1] Xen PV support for hugepages

[Jeremy Fitzhardinge <jeremy@goop.org>]

Dave McCracken wrote:
> On Thursday 06 November 2008, Jan Beulich wrote:
>   
>>> This is the point of having hugepages a command line option.  It should
>>> only be turned on if you intend to run guests who enforce the alignment
>>> rules.
>>>       
>> You mean 'if you intend to run *only* guests ...', including dom0. Any
>> guest unaware of the connection of X86_FEATURE_PSE and the need to create
>> contiguous 2M chunks would fail, and any guest not having I/O memory
>> assigned would never manage to create such chunks.
>>     
>
> Hmm... Sounds like a per-domain flag would be a good solution here.  I'll look 
> into it.
>   

I think a guest should explicitly enable large page support via 
something like vmassist.  In other words:

    * keep PSE clear - you are not implementing something similar to PSE
    * export the ability to map large pages via a feature flag or something
    * require guests to explicitly enable large page support


I'm sceptical about the value of this patch as it stands.  We already 
have a vast number of operating modes and combinations, and I think we 
need to have a pretty high bar before adding any more.  If this work 
arranged things so that we could just set PSE - and thereby make 
supporting it on the guest side much simpler - then I think the decision 
to include it would be easier.

In practical terms, I think this means that we need to modify Xen and 
the domain builder to *always* allocate memory to guests in 2M contig 
chunks, and then sort out all the tricky details ;)  In practice, 
however, we would lose a lot of the benefit of doing this we'd get from 
the native kernel, because all the RO pagetable mappings would chop up 
the linear and kernel maps so much that we'd not get much chance to use 
the 2M mappings.  It would, however, make hugetlbfs work without much drama.

    J