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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 12/06/2017 03:19 PM, Julien Grall wrote:
> Hi Konrad,
> 
> On 12/06/2017 03:10 PM, Konrad Rzeszutek Wilk wrote:
>> .snip..
>>> The suggested policy is based on the KVM one:
>>>     - If we trap a S/W instructions, we enable VM trapping (e.g
>>> HCR_EL2.TVM) to
>>> detect cache being turned on/off, and do a full clean.
>>>     - We flush the caches on both caches being turned on and off.
>>>     - Once the caches are enabled, we stop trapping VM instructions.
>>>
>>> Doing a full clean will require to go through the P2M and flush the
>>> entries
>>> one by one. At the moment, all the memory is mapped. As you can imagine
>>> flushing guest with hundreds of MB will take a very long time (Linux
>>> timeout
>>> during CPU bring).
>>
>> Yikes. Since you mention 'based on the KVM one' - did they solve this
>> particular
>> problem or do they also have the same issue?
> 
> KVM is using populate on demand by default.

If I understand properly, it's probably more accurate to say that KVM
uses "allocate on demand".  The complicated part of populate-on-demand
is the fact that it's not allowed to allocate anything.

 -George

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