Re: [PATCH] tools/libxl: don't allow IOMMU usage with PoD

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Feb 03, 2022 at 12:55:56PM +0100, Jan Beulich wrote:
> On 03.02.2022 12:06, Roger Pau Monne wrote:
> > Prevent libxl from creating guests that attempts to use PoD together
> > with an IOMMU, even if no devices are actually assigned.
> > 
> > While the hypervisor could support using PoD together with an IOMMU as
> > long as no devices are assigned, such usage seems doubtful. There's no
> > guarantee the guest has ballooned down enough memory for PoD to no
> > longer be active, and thus a later assignment of a PCI device to such
> > domain could fail.
> 
> That's not a precise description of the constraint: The guest ballooning
> down enough only means entries == cache, but for device assignment we
> need entries == 0 (and a guarantee that no new entries can appear, but I
> think this is already the case once a guest was launched).

Would you be OK with:

"While the hypervisor could support using PoD together with an IOMMU as
long as no devices are assigned, such usage seems doubtful. There's no
guarantee the guest has PoD no longer be active, and thus a later
assignment of a PCI device to such domain could fail."

By "PoD no longer be active" meaning cache == entries == 0.

Thanks, Roger.