From: Juan Quintela <quintela@redhat.com>
In-Reply-To: <002b01d110c0$4a5c8e40$df15aac0$@samsung.com> (Pavel Fedin's
	message of "Tue, 27 Oct 2015 17:03:42 +0300")
References: <012c01d110a6$cd61cf90$68256eb0$@samsung.com>
	<87k2q8e8d4.fsf@neno.neno>
	<002b01d110c0$4a5c8e40$df15aac0$@samsung.com>
Date: Wed, 28 Oct 2015 10:58:33 +0100
Message-ID: <877fm7b9h2.fsf@neno.neno>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [Qemu-devel] [PATCH] migration:
	Introduce	migration_in_completion()
Reply-To: quintela@redhat.com
To: Pavel Fedin <p.fedin@samsung.com>
Cc: 'Amit Shah' <amit.shah@redhat.com>, qemu-devel@nongnu.org, 'Luiz Capitulino' <lcapitulino@redhat.com>

Pavel Fedin <p.fedin@samsung.com> wrote:
>  Hello!
>
>> Applied, thanks.
>
>  Thank you.
>
>> I wonder who it has never been needed before.  BTW, what is ITS?
>
>  ITS is Interrupt Translation Service. It is used by ARM64
> architecture. In a short, it is a special hardware which performs
> table-based lookup of MSI IDs and their destination CPUs, and routes
> them. There can be lots of CPUs and lots of interrupts, so
> these tables are large and are held in a system RAM.
>  qemu implementation of this thing needs to be able to flush its
> internal cached state into memory for live migration. This has to
> be done during final stage, right after CPUs have been stopped,
> because right after this point migration code will recheck RAM
> status and send out modified data for the last time.
>  The implementation itself has to wait, because the necessary KVM API
> is also not ready yet, but i prefer to upstream as much of the
> infrastructure as possible, because: a) we want to reduce amount of
> out-of-tree patches for our project; b) these things affect qemu
> core code, and upstreaming them makes sure that we do it right, and
> our out-of-tree code will not diverge from the upstream too
> much.

Power people have a similar problem with its hashed page tables, they
integrated their own save_live implementation because they are too big
for the last stage.  You can look there for inspiration.

How big are we talking here?

Thanks, Juan.

>
> Kind regards,
> Pavel Fedin
> Expert Engineer
> Samsung Electronics Research center Russia