Re: [Xen-devel] [PATCH v4 10/12] IOMMU: introduce IOMMU_MIXED config option

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.10.18 at 12:38, <julien.grall@xxxxxxx> wrote:
> On 02/10/2018 11:18, Jan Beulich wrote:
>> ARM is intended to gain support for heterogeneous IOMMUs on a single
>> system. This not only disallows boot time replacement of respective
>> indirect calls (handling of which is the main goal of the introduction
>> here), but more generally disallows calls using the iommu_ops() return
>> value directly - all such calls need to have means (commonly a domain
>> pointer) to know the targeted IOMMU.
>> 
>> Disallow all hooks lacking such context for the time being, which in
>> effect is some dead code elimination for ARM. Once extended suitably,
>> individual of these hooks can be moved out of their guards again in the
>> future.
> 
> While in theory it is possible to have platform with hetereneous IOMMUs. 
>   I don't see such such support coming in Xen for the foreseeable 
> future. Note that even Linux does not support such case.
> 
> This patch is going to make more complicate to unshare page-tables as 
> now we would need to care of mixed case. So I would rather not set 
> IOMMU_MIXED on Arm until we have a use case for it.

Interesting. I essence this is the exact opposite of what you've
told me when I inquired about indirect call patching of the IOMMU
code. The sole purpose of this new option is to have a key off of
which I can tell whether to use patchable indirect calls or plain
ones.

I'm also not following how this change would complicate anything
for you. There's effectively no change for ARM, except for some
dead code not getting built anymore.

Jan



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