Re: [Xen-devel] Qemu upstream drive question

[Daniel Castro <evil.dani@xxxxxxxxx>]

On Fri, Feb 17, 2012 at 5:42 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 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:
> docs/misc/vbd-interface.txt.
>
>> 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
>                docs/misc/vbd-interface.txt,
>              * "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).
>
> Ian.
Thanks a million Ian. Perfectly clear now, I will get on to it then.
>
>



-- 
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| +---------------------------------+ | This space intentionally blank
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| |   | Daniel Castro,                |
| |   | Consultant/Programmer.|
| |   | U Andes                         |
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