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

[Andrii Tseglytskyi <andrii.tseglytskyi@xxxxxxxxxxxxxxx>]

Hi Stefano,

On Tue, Jul 22, 2014 at 3:42 PM, Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> wrote:
> 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.

Sorry, I didn't get the point here. Looks like you are talking about
TLB flush initiated by Xen? Am I right?

-- 

Andrii Tseglytskyi | Embedded Dev
GlobalLogic
www.globallogic.com

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