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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 16 Jul 2014, Ian Campbell wrote:
> 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.
 
I guess it depends on how often Xen or the Xen tools are going to change
the p2m for a running domain. For example changing one page mapping a
couple of times a second would be very expensive if we have to rebuild
the remoteproc pagetables every time.

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