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Re: [Xen-devel] [PATCH 3/6] remove late (on-demand) construction of IOMMU page tables

On 12.08.2019 18:26, Paul Durrant wrote:
-----Original Message-----

On 30.07.2019 15:44, Paul Durrant wrote:
NOTE: This patch will cause a small amount of extra resource to be used
        to accommodate IOMMU page tables that may never be used, since the
        per-domain IOMMU flag enable flag is currently set to the value
        of the global iommu_enable flag. A subsequent patch will add an
        option to the toolstack to allow it to be turned off if there is
        no intention to assign passthrough hardware to the domain.

In particular if the default of this is going to be "true" (I
didn't look at that patch yet, but the wording above makes me
assume so), in auto-ballooning mode without shared page tables
more memory should imo be ballooned out of Dom0 now. It has
always been a bug that IOMMU page tables weren't accounted for,
but it would become even more prominent then.

Ultimately, once the whole series is applied, then nothing much should change 
for those specifying
passthrough h/w in an xl.cfg. The main difference will be that h/w cannot be 
passed through to a
domain that was not originally created with IOMMU pagetables.
With patches applied up to this point then, yes, every domain will get IOMMU 
page tables. I guess I'll
take a look at the auto-ballooning code and see what needs to be done.

Ok, I've had a look...

I could make a rough calculation in libxl_domain_need_memory() based on
the domain's max_memkb and an assumption of a 4 level translation with
512 PTEs per level, or would prefer such guestimation to be overridable
using an xl.cfg parameter in a broadly similar way to shadow_memkb?

I've always been thinking that for the non-shared case the amount
reserved for page tables could simply be doubled.


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