Re: [Xen-devel] [PATCH V5 2/3] x86/mm: allocate logdirty_ranges for altp2ms

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 11/13/18 8:43 PM, Razvan Cojocaru wrote:
> On 11/13/18 7:57 PM, George Dunlap wrote:
>> On 11/11/18 2:07 PM, Razvan Cojocaru wrote:
>> At the moment we essentially have two "init" states:
>> * After domain creation; altp2m structures allocated, but no rangesets, & c
>> * After being enabled for the first time: rangesets mirroring hostp2m,
>> p2m_init_altp2m_ept() initialization done
>>
>> Is there any particular reason we allocate the p2m structures on domain
>> creation, but not logdirty range structures?  It seems like allocating
>> altp2m structures on-demand, rather than at domain creation time, might
>> make a lot of the reasoning here simpler.
> 
> I assume that this question is not addressed to me, since I'm not able
> to answer it - I can only assume that having less heap used has been
> preferred.

Actually I now realize that you're asking why the hostp2m rangeset is
created via paging_domain_init() in arch_domain_create() (so immediately
on domain creation) while I'm allocating the altp2m rangesets on altp2m
init.

I'm doing that to save memory, since we can have MAX_ALTP2M altp2ms
(which is currently 10), and only two active altp2ms - that means that I
would allocate 10 rangesets and only use two. In fact we're currently
only using 2 altp2ms and the hostp2m for our #VE work. That saves the
space required for 8 rangesets. If that's not much, or if you think that
the benefits of allocating them early outweigh the costs we can switch
to allocating them on domain creation, like the hostp2m, and perhaps
always keeping them in sync.


Thanks,
Razvan

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