Re: Summary of LPC guest MSI discussion in Santa Fe

[Alex Williamson]

On Thu, 10 Nov 2016 15:40:07 +0100
Joerg Roedel <joro@xxxxxxxxxx> wrote:

> 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.

Right, I'm not advocating a paravirt ballooning approach, just pointing
out that they're both terrible, just in different ways.  Thanks,

Alex