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From: Ian Campbell <ian.campbell@citrix.com>
Date: Wed, 14 Oct 2015 12:27:52 +0100
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Subject: Re: [Qemu-devel] [Xen-devel] Question about xen disk unplug support
 for ahci missed in qemu
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To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Kevin Wolf <kwolf@redhat.com>
Cc: qemu-block@nongnu.org, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>, Fabio Fantoni <fabio.fantoni@m2r.biz>, Anthony.Perard@citrix.com, John Snow <jsnow@redhat.com>

On Wed, 2015-10-14 at 12:06 +0100, Stefano Stabellini wrote:
> > Can't you just teach SeaBIOS how to deal with your PV disks and then
> > only add that to your VM and forget about IDE/AHCI? I mean, that's how
> > it's done for virtio-blk, and it doesn't involve any insanities like
> > ripping out non-hotpluggable devices.
> 
> Teaching SeaBIOS to deal with PV disks can be done, in fact we already
> support PV disks in OVMF. It is possible to boot Windows with OVMF
> without any IDE disks (patch pending for libxl to create a VM without
> emulated IDE disks).

One stumbling block in the past has been how to know when the PV drivers in
the BIOS are no longer required, such that the ring can be torn down and/or
the connection etc handed over to the OS driver.

I think we deal with this in OVMF using ExitBootServices? (TBH I'm not sure how). AFAIK the BIOS interfaces do not have anything as reliable as that.

How does virtio deal with this in the BIOS case?

Ian.