Re: [Xen-devel] Re: [PATCH] EPT: Only sync pcpus on which a domain's vcpus might be running

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 18/09/2009 10:09, "George Dunlap" <dunlapg@xxxxxxxxx> wrote:

> Just making sure I understand the failure mode... the risk is that if
> we change the p2m for this domain while a vcpu is not running on that
> core, and it's scheduled there again, it may have stale mappings that
> get used (tagged by the EPT base pointer).

Correct.

> Hmm... any idea the performance implications of flushing the EPT
> mappings when switching out?  The main performance problem I need to
> fix is the ballooning case.  Without this patch, on my 2x4x2 Nehalem
> box, a single-vcpu guest ballooning from 16GiB down to 8GiB for an
> initial boot takes 12-15 minutes; with the patch, it takes around 40
> seconds.

How bad is it is you don't flush at all? 40s is still not great and maybe we
could batch the INVEPT invocations? That might also greatly reduce the cost
of the current naïve flush-all implementation, to a point where it is
actually acceptable.

> Perhaps we could start with the flush-ept-on-context-switch-out, and
> do some performance testing later to see if it's worth maintaining a
> "might-need-an-ept-flush" mask.

Well, that's a possible alternative, yes. Just needs a bit of care in
implementation but it wouldn't need much extra code.

 -- Keir



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