Re: [Qemu-devel] On block interface types in general, IF_AHCI in particular

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> The main drawback of if=ahci is, as pointed out by Markus, that you
> would have to provide both controllers on-board.  I think a real ICH9
> has the compatibility IDE controller on a separate PCI address from the
> SATA controller, so creating both of them is not really out of question.

I think this can be turned on and off by the bios.  When configured in
ahci mode there is no IDE controller.  It shows up when you switch to
IDE in the bios though.

[ side note: I suspect this could be the reason why the *BSDs don't
             work with our AHCI emulation, might be they try to look
             at the ich9-specific registers which control ahci/ide mode
             and we don't emulate that properly. ]

> Obvious disadvantage, it would depart from real hardware.  Linux should
> not care, not sure about SeaBIOS and Windows.

> At the same time, if all we want is a quick way to switch between IDE
> and AHCI, we could just use machine types.  So another proposal is to
> have two machine types, one for ICH9-IDE (pc, the default), one for
> ICH9-AHCI (q35), one for PIIX3-IDE (piix3).  Each QEMU release would add
> (up to) three machine types.

I'd tend to just point people to pc-piix for guests too old to be
shipped with ahci drivers and be done with it.  Chances are high that
they have more issues with q35 anyway.

cheers,
  Gerd