Re: RE: Tmem vs order>0 allocation, workaround RFC

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Feb 16, 2010 at 07:05:48AM -0800, Dan Magenheimer wrote:
> Hi Jan --
> 
> Thanks for thinking about this.
> 
> > may not work well: When you have 1Tb, you'd reserve 8G, making Dom0
> > single-page-below-4G-allocations impossible (unless dom0_mem= was
> > used) if I read the logic correctly.
> 
> Good point.  But tmem doesn't work very well at all if dom0_mem
> isn't set as dom0 is hogging all the spare memory in the system
> so only fallow memory reclaimed from selfballooning domains
> can be used by tmem.
> 
> Under what circumstances does dom0 require single-page-below-4G
> allocations?  Is it only for bounce buffers for PCI passthrough
> of old devices with 32-bit addressing limitations?  Or am I
> missing a much more common case?  (I think it's important to

The software IO TLB is initialized unconditionally if no IOMMUs are found.
This is a 64MB + 32Kb chunk of memory that is exchanged with Xen to make sure
it is under the 32-bit mark.

> enumerate and understand -- and document -- all special needs
> of memory pages as Xen has been fairly careless/lucky with
> fragmentation so far, but with all the memory optimization
> technologies in 4.0, we need to root out all the cases.)
> 
> Thanks,
> Dan
> 
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