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.

So if I drop this here, how would you want iommu_get_ops()
get handled on Arm (patch 11)? Right now I'd mean to leave it
alone, but it could also be switched to the (new) x86 way (but
that would then perhaps make mixed mode introduction more
difficult down the road), allowing to get away with fewer
#ifdef-s.

Jan



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