Re: Summary of LPC guest MSI discussion in Santa Fe

[Joerg Roedel]

On Wed, Nov 09, 2016 at 01:01:14PM -0700, Alex Williamson wrote:
> Well, it's not like QEMU or libvirt stumbling through sysfs to figure
> out where holes could be in order to instantiate a VM with matching
> holes, just in case someone might decide to hot-add a device into the
> VM, at some point, and hopefully they don't migrate the VM to another
> host with a different layout first, is all that much less disgusting or
> foolproof. It's just that in order to dynamically remove a page as a
> possible DMA target we require a paravirt channel, such as a balloon
> driver that's able to pluck a specific page.  In some ways it's
> actually less disgusting, but it puts some prerequisites on
> enlightening the guest OS.  Thanks,

I think it is much simpler if libvirt/qemu just go through all
potentially assignable devices on a system and pre-exclude any addresses
from guest RAM beforehand, rather than doing something like this with
paravirt/ballooning when a device is hot-added. There is no guarantee
that you can take a page away from a linux-guest.


	Joerg