Re: [PATCH] Xen/i386 cleanups - AGP bus/phys cleanups

[Keir Fraser <Keir.Fraser@cl.cam.ac.uk>]

On 19 Mar 2005, at 10:56, Christian Limpach wrote:

>> For this specific case there may be another resolution but could
>> you please, please look at marking the missing pages PG_reserved
>> and not hacking phys_to_virt.
>>
>> At this point anything short of explicitly introducing an intermediate
>> step say virt_to_logical() logical_to_virt() will be extremely
>> confusing and lead to very hard to spot bugs.  Silently changing
>> the semantics of functions is bad.
>
> We also use the additional level of indirection to implement suspend/
> resume and relocation of virtual machines between physical machines  --
> you won't get the same sparse allocation of memory on the target 
> machine.
> Also, this will make it much easier to support hot plug memory at the
> hypervisor level since it will be able to substitute memory with very
> little support from the OS running in the virtual machine.

Also, more generally, I don't believe Linux would deal well with a 
highly fragmented memory map. I wonder how far Linux would boot if you 
PG_reserved every other page? We'd also need to deal with 
virtual<->lowmem not being a 1:1 mapping (at least for kernel code and 
data, as at least that obviously needs to be contiguous in virtual 
space).

  -- Keir