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Re: [PATCH] tools/libxl: don't allow IOMMU usage with PoD
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 3 Feb 2022 12:55:56 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 03 Feb 2022 11:56:09 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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).
(FWIW the wording in code comment and log message read fine to me.)
Jan
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