[Xen-devel] [RFC] new totalmem= boot parameter

[Sarina Canelake <sarina.canelake@xxxxxxxxxx>]

We have a need for ensuring the total RAM available to [Xen / the kernel] at 
boot is X MB because there are situations in which you wish to limit the amount 
of RAM available to a box. The existing mem= option doesn't work because it 
limits the maximum physical address, NOT the amount of available RAM. Many, if 
not all, systems contain a substantial memory hole below 4 Gb, typically a 0.5 
or 1 Gb hole from 3-4 Gb. Thus, on a system with 6 Gb of RAM, requesting 
mem=4096M will yield a box with maximum physical address in the 4 Gb 
neighborhood but perhaps only 3 or 3.5 actual gigs of RAM available.

So I propose to add a new kernel argument, which I call totalmem, that limits 
the total amount of memory available, ignoring holes. So requesting 
totalmem=4096M in the preceding example, the maximum physical address may 
extend much higher depending on the size of the memory map hole. An example 
patch is attached; it doesn't apply directly to xen-unstable but should provide 
an example implementation for discussion; I implemented it to conform to the 
changes made to the clip_to_limit function in xen-unstable. 

Alternately, we could modify mem= to do this; according to documentation, it 
seems that the behavior of totalmem reflects the designer's goal for mem. 
However, I would be worried about issues of backwards compatibility, as people 
may already be exploiting the current functionality of mem= to directly cut off 
the address space.  Comments? (P.S.  This is my first post to xen-devel so 
please be kind :)

Thanks,
Sarina

Attachment: xen-add-totalmem-kernel-arg.patch
Description: Text Data

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