Re: [Xen-devel] RE: Ballooning up

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 On 09/14/2010 08:06 AM, Dan Magenheimer wrote:
>> true, if you don't intend to balloon up, there's no point wasting
>> memory on unused page structures.
> I think this is the key.  If dom0_mem is NOT specified, dom0
> launches with (essentially) all the physical memory of the
> machine, page tables are allocated in dom0 to map all of physical
> memory, and auto-ballooning is necessary to launch guests.
>
> If dom0_mem IS specified, it is often a much smaller number
> than size of physical memory; why waste ~1.5% of physical memory
> on page structures that will never be used?
>
> If someone wants to add an option to augment dom0_mem to allow
> memory-up-ballooning of dom0 above dom0_mem (and can justify
> a reason why some user might ever use this functionality),
> that's fine.  But let's not change the definition of the
> dom0_mem option just because a bug fix happens to make it
> possible.

Technically (pedantically), the meaning of dom0_mem is unchanged - it
sets the initial number of pages given to the domain, and is
functionally identical to the normal "memory" parameter in a domU config
file.  The difference is that we're now paying attention to the E820
map, which is set by maxmem= in domU, but is the hardware/BIOS one in dom0.

I'm not sure what I'm doing that's different to the xenolinux kernels; I
guess they hack up the whole memory init path more aggressively.  But
the pvops behaviour is more or less the straightforward outcome of
looking at the Xen-provided E820 and reserving the gaps between the
actual page count and the memory described therein.

    J

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