Re: [PATCH Xen-unstable] Balloon down memory to achive enough DMA32 memory for PV guests with PCI pass-through to succesfully launch.

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

> No, you probably misunderstood (and I probably implied to much in my
> response): On a system with more than 168G, just ballooning out
> arbitrary memory from Dom0 in order to start a 32-bit pv DomU won't
> guarantee that the domain can actually start, as memory beyond the
> 128G boundary is unusable there for such guests.

I see what you mean.
> 
> Conceptually, ballooning out arbitrary amounts of memory to find a
> certain amount below 4G is identical to ballooning out more than the
> amount a guest needs in order to find as much as it needs below 128G.

Perhaps an extra mechanism in the balloon driver in the Dom0 kernel
to only give back to Xen pages below the 4GB mark? And if it
fails return a failure back to Xend?

That would solve the problem of ballooning out more than needed and
would eliminate the "oh-lets balloon out 2 more MBs and see if that
did it".

.. snip ..
> >Since the SWIOTLB size is determined by the 'swiotlb' argument passed to
> >the guest, what if we scanned for that and if it has a number, calculate
> >how much memory that amounts to and use that value? The default still being at 64MB.
> 
> That might be an option, but is very Linux-centric. I think the amount should
> be configurable per guest if something like this is being done at all.

How about both? Have a 'pci_mem' argument to set the default and also
check the swiotlb argument in 'extra.' If there is a 'swiotlb' argument in
'extra' use that value (computed for kilobytes of course).