Re: [PATCH] x86: use 'dom0_mem' to limit the number of pages for dom0

[David Vrabel <david.vrabel@citrix.com>]

On 16/08/11 01:31, Konrad Rzeszutek Wilk wrote:
> On Mon, Aug 15, 2011 at 07:08:33PM +0100, David Vrabel wrote:
>> x86: use 'dom0_mem' to limit the number of pages for dom0
>>
>> Use the 'dom0_mem' command line option to set the maximum number of
>> pages for dom0.  dom0 can use then use the XENMEM_maximum_reservation
>> memory op to automatically find this limit and reduce the size of any
>> page tables etc.
> 
> So .. I was actually curious about this -how did this use to work
> with 2.6.18? In there, if you did 'dom0_mem=max:2GB' it would limit the
> amount of memory - so how does it do that?

I don't know. I've not really looked at how the older kernels did it.

>> diff -r 8d6edc3d26d2 -r 1ec6c392d40f xen/arch/x86/domain_build.c
>> --- a/xen/arch/x86/domain_build.c	Sat Aug 13 10:14:58 2011 +0100
>> +++ b/xen/arch/x86/domain_build.c	Mon Aug 15 11:42:37 2011 +0100
>> @@ -254,6 +254,8 @@ static unsigned long __init compute_dom0
>>      }
>>  #endif
>>  
>> +    d->max_pages = min(max_pages, avail);
>> +
> 
> don't you need the amount of p2m as well? Or is that all limited
> by d->max_pages?

Do you mean the initial p2m supplied by Xen?  I think that just covers
the initial pages.

The p2m structure maintained by the kernel appears to be dynamically
allocated based on what mappings are required.

David