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Re: [Qemu-devel] balloon vs postcopy migrate

From: Pankaj Gupta
Subject: Re: [Qemu-devel] balloon vs postcopy migrate
Date: Tue, 3 Feb 2015 12:36:32 -0500 (EST)

> Hi,
>   Andrea pointed out there is a risk that a guest inflating its
> balloon during a postcopy migrate could cause us problems, and
> I wanted to see what the best way of avoiding the problem was.
> Guests inflating there balloon cause an madvise(MADV_DONTNEED) on
> the host, marking pages as not present, that will potentially trigger
> a userfault, that we are using in postcopy to detect pages that need
> to be fetched from the source.
> In theory, at the moment guests *should* only ask for a balloon
> inflation if they've been asked to do so by the host; however there
> are no guards for that, and it's been suggested giving the
> guest more freedom might be a good idea anyway.
> My alternatives seem to be:
>    1) Stop servicing the message queue from the guest so
>      that we just don't notice the inflate messages until
>      afterwards.  (Easy for Qemu, not sure how the guests
>      will like an unserviced queue).
>    2) I could keep servicing the queue and ignore the messages
>      (Easy for everyone, not very nice in actual used memory -
>       does it cause any long term problems other than that?)
>    3) I could keep servicing the queue but put the messages
>      in a list somewhere that replay after migrate has finished.
>      (That list sounds bounded only in a very large way?)
> Thoughts?

Can we have some global flag somewhere when Post copy is ON/active.
And we can ignore or defer only inflate/ballon messages/commands while 
servicing the commands with some warnings.

Just my thought on logic. Not sure if I am missing some background here.

> Dave
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK

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