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Date: Wed, 31 Oct 2012 15:09:09 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] On block interface types in general,
	IF_AHCI in particular
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To: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, Jason Baron <jbaron@redhat.com>, qemu-devel@nongnu.org, Alexander Graf <agraf@suse.de>, Gerd Hoffmann <kraxel@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

Il 31/10/2012 14:58, Markus Armbruster ha scritto:
> So by default, you can have only two fast devices, and you get them only
> if you know that you need to use unit=4 and unit=5.

Or "-m q35".

> Naive usage, including -hda, gets IDE compatibility mode, which is slower.
> 
> I think this is robbing Peter to pay Paul.

Everything is.

> Peters getting robbed:
> 
> * New guests that would deal just fine with q35 including AHCI lose:
>   they get "upgraded" to AHCI in IDE compatibility mode, which is slower
>   than the real thing.

But also slower than virtio. :)

> * Existing guests that could deal with the hardware upgrade from i440FX
>   to q35 just fine lose the same way.
> 
> Pauls getting paid:
> 
> * Existing guests that can deal with the upgrade *except* for AHCI
>   *continue to work.
> 
> And there are Pats who still won't get paid:
> 
> * Existing guests that can't deal with the chipset changing under them
>   still break.

Do we know of any?

> If we make q35 the default and q35 with compat IDE an option, then the
> Peters just work, and the Pauls and the Pats break on upgrade until you
> specify a machine type that works.

Is there anything that prevents us from doing it later?

Paolo