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

On 12/06/2017 03:24 PM, George Dunlap wrote:
On 12/06/2017 03:19 PM, Julien Grall wrote:
Hi Konrad,

On 12/06/2017 03:10 PM, Konrad Rzeszutek Wilk wrote:
The suggested policy is based on the KVM one:
     - If we trap a S/W instructions, we enable VM trapping (e.g
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
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
during CPU bring).

Yikes. Since you mention 'based on the KVM one' - did they solve this
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.

Hmmm yes. You are right on the wording.


Julien Grall

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