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

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 02/10/2018 11:42, Jan Beulich wrote:
> > > > 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 don't think I am saying the opposite. I opened the door for mixed use 
case. It does not mean, I want to see half of the helpers dropped on Arm 
because you want them to be patchable. There are other way to do it.
> 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.
Well, with your patch free_page_table, resume & co are under 
!IOMMU_MIXED. So they are unusable on Arm until we effectively rework
the code to handle mixed case. More likely those callback will be 
necessary on Arm before mixed case. So I don't think this patch as such 
is what we want for Arm at the moment.
I would much prefer if you drop IOMMU_MIXED and implement iommu_{v,}call 
on Arm as a iommu_ops->fun(...) (i.e no alternative for now). So call 
are still not patchable for Arm, yet we still have access to all IOMMU 
functions.
Cheers,

--
Julien Grall

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