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Re: [Xen-devel] Qemu upstream drive question

VOn Thu, 2012-02-16 at 22:52 +0000, Daniel Castro wrote:
> On Thu, Feb 16, 2012 at 8:13 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Thu, 2012-02-16 at 10:53 +0000, Daniel Castro wrote:
> >> Hello All,
> >>
> >> When a HVM guest is started upstream qemu presents the emulated drives
> >> as PCI devices or ata drives?
> >
> >> If more than drive is present how can I know which drive is which in 
> >> xenstore?
> >
> > Emulated devices do not appear in xenstore.
> >
> > Some PV devices may have an emulated equivalent but the rule of thumb is
> > that an OS should only use either PV or Emulated devices and not mix and
> > match.
> >
> > In Linux we implement this by requiring that the emulated devices are
> > unplugged before we will use the PV path.
> >
> > In principal you can infer which Emulated device might matches a PV
> > device, the file docs/misc/vbd-interface.txt in the xen tree gives some
> > hints about this, but basically PV disks 0..3 can correspond to the
> > first four IDE devices (primary-master, primary-slave, secondary-master
> > & secondary-slave).
> >
> > I presume you are asking in the context of SeaBIOS. In that context you
> > could argue against unplugging because the guest OS may want to use
> > emulated devices, however I would suggest that for simplicity for time
> > being you just unplug the emulated devices before you enable the PV
> > path.
> Thanks for the response Ian, I am not sure I understand you. What I do
> is something similar to this:
> Find the emulated devices.

Why? You don't need these in your Xen code.

If anything is going to enumerate these devices it will be the existing
SeaBIOS code which drive the emulated devices. There is no need for you
to touch that.

>  ATA Devices are the drives and the PCI device is for what?

You are referring to the Xen platform device (5853:0001)?. This device
has nothing to do with the emulated disks.

The platform device just serves as a useful point to hook the Xen driver
loading off since many OSes handle autoloading of PCI drivers well so we
use it to trigger the loading of Xen drivers.

It has some secondary purposes such as the IRQ associated with this
device can also be used to inject event channels and the unplug protocol
happens via I/O ports on this device. 

> Find if the device is a disk. (in the context of a vbd the xenstore id
> corresponds to what?)

The meaning of the VBD number (e.g. 832 and 985 in your examples below)
is described by the document I pointed you at in my last mail:

> If disk, then find corresponding entry on xenstore. Example
> device/vbd/832, device/vbd/985

You should enumerate (using XS_DIRECTORY) the device/vbd directory. Each
entry you find in there is a disk. There is no need for you to go
anywhere near the emulated disk devices to do this, nor to establish any
correspondence with the emulated devices. You know they are all vbds
because you found them in device/vbd.

The sequence of events should be:

      * Notice that you are running under Xen and that the platform PCI
        device is present. You most likely want to do this by using
        pci_find_init_device() to register a callback for the platform
        device. If you never get a callback then there is nothing for
        you to do and you should assume standard SeaBIOS drivers etc
        will take care of the emulated devices.
      * Unplug all of the emulated devices per the protocol defined in
        docs/misc/hvm-emulated-unplug.markdown. You do not need to
        enumerate the emulated devices to do this. If this fails then
        abort and assume standard SeaBIOS drivers etc will take care of
        the emulated devices.
      * Enumerate the contents of device/vbd in xenstore, for each disk:
              * parse the disk and partition number per
              * "Write device details to xenstore -rings, ref, etc" and
                do the necessary xenbus state transitions to get into
                state 4 (XenBusConnected)
              * create and register the appropriate disk device
                structures with SeaBIOS.

In the future once all this is working then there may be extensions such
as not unplugging devices and ensuring the safe coexistence of the
emulated and PV devices but you can leave that to one side for now since
there are other issues with doing that (c.f. previous discussions about
how to safely shutdown the PV devices after SeaBIOS is finished).


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