[Xen-devel] RE: Tmem vs order>0 allocation, workaround RFC

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

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
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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