Re: [Qemu-devel] [PATCH] `qdev_free` when unplug a pci device

[William Dauchy <wdauchy@gmail.com>]

Hi Markus,

On Wed, Feb 23, 2011 at 9:30 AM, Markus Armbruster <armbru@redhat.com> wrote:
> I don't think this patch is correct.  Let me explain.
>
> Device hot unplug is *not* guaranteed to succeed.
>
> For some buses, such as USB, it always succeeds immediately, i.e. when
> the device_del monitor command finishes, the device is gone.  Live is
> good.
>
> But for PCI, device_del merely initiates the ACPI unplug rain dance.  It
> doesn't wait for the dance to complete.  Why?  The dance can take an
> unpredictable amount of time, including forever.
>
> Problem: Subsequent device_add can fail if it reuses the qdev ID or PCI
> slot, and the unplug has not yet completed (race condition), or it
> failed.  Yes, Virginia, PCI hotplug *can* fail.
>
> When unplug succeeds, the qdev is automatically destroyed.
> pciej_write() does that for PIIX4.  Looks like pcie_cap_slot_event()
> does it for PCIE.
>
> Your patch adds a *second* qdev_free().  No good.

Thanks for the complete explanation. I now understand my mistake and
find out why it wasn't working as expected.
On a Linux virtual machine I forgot to load the `acpiphp` module which
makes the pci device disappear when doing a detach. I thought that,
even without this module the detach should have freed the
corresponding device on qemu side, regardless if the virtual machine
was supporting or not acpi signals.
Please ignore my patch.

Regards,
-- 
William