Re: [Xen-devel] [RFC] xen/arm: Handling cache maintenance instructions by set/way

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.12.17 at 14:52, <julien.grall@xxxxxxxxxx> wrote:
> On 06/12/17 17:49, George Dunlap wrote:
>> Do you want to reset the p2m multiple times?  I thought the goal was
>> simply to keep the amount of p2m space you need to flush to a minimum;
>> if you expect the memory which has been faulted in by the *last* flush
>> to be relatively small, you could just always flush all memory that had
>> been touched to that point.
>> 
>> If you *do* need to go through the p2m multiple times, then
>> misconfiguration is a much better option than PoD.  In PoD, once a page
>> has data on it, it can't be removed from the p2m anymore.  For the
>> misconfiguration technique, you can go through and misconfigure the
>> entries in the top-level p2m table as many times as you want.  The whole
>> reason for doing it on x86 is that it's a relatively lightweight
>> operation: we use it to modify MMIO mappings, to enable or disable
>> logdirty for migrate, &c.
> 
> Does this also work when you share the page-tables with the IOMMU? It 
> just occurred to me that for both PoD and "misconfigured bits" we would 
> get into trouble because page-tables are shared with the IOMMU.

PoD and IOMMU are incompatible on x86 at present.

The bits we use for "mis-configuring" entries are ignored by the IOMMU,
which is not a problem since all we use this approach for (right now) is
to update the memory type (i.e. cacheability) for possibly huge ranges.

Jan


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