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Re: [Xen-devel] [PATCH v02 1/7] arm: introduce remoteprocessor iommu module

On Fri, 2014-07-04 at 14:59 +0100, Stefano Stabellini wrote:
> On Thu, 26 Jun 2014, Andrii Tseglytskyi wrote:
> > This is a fisrst patch from patch series which was
> > developed to handle remote (external) processors
> > memory management units. Remote processors are
> > typically used for graphic rendering (GPUs) and
> > high quality video decoding (IPUs). They are typically
> > installed on such multimedia SoCs as OMAP4 / OMAP5.
> > 
> > As soon as remoteprocessor MMU typically works with
> > pagetables filled by physical addresses, which are
> > allocated by domU kernel, it is almost impossible to
> > use them under Xen, intermediate physical addresses
> > allocated by kernel, need to be translated to machine
> > addresses.
> > 
> > This patch introduces a simple framework to perform
> > pfn -> mfn translation for external MMUs.
> > It introduces basic data structures and algorithms
> > needed for translation.
> > 
> > Typically, when MMU is configured, some it registers
> > are updated by new values. Introduced frameworks
> > uses traps as starting point of remoteproc MMUs
> > pagetables translation.
> > 
> > Change-Id: Ia4d311a40289df46a003f5ae8706c150bee1885d
> > Signed-off-by: Andrii Tseglytskyi <andrii.tseglytskyi@xxxxxxxxxxxxxxx>
> There is one problem with this patch: you need to find a way to "pin"
> the p2m entries for the pfns and mfns found in the pagetables translated
> by remoteproc_iommu.c.  Otherwise Xen might decide to change the pfn to
> mfn mappings for the domain, breaking the pagetables already translated
> by remoteproc_iommu.c.  pfn to mfn mappings are not guaranteed to be
> stable. At the moment Xen on ARM wouldn't change pfn to mfn mappings
> under the guest feet, but features like memory sharing or swapping,
> supported on x86, could cause it to happen.

I've not fully grokked this patch but wouldn't it be easier and better
to have a hook to cause the remoteproc to throw away and rebuild its
mappings when this happens? It could be called from e.g. the TLB flush
associated with the p2m changing, I think.


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