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Re: [Qemu-devel] [PATCH 2/2] v2 Fix Block Hotplug race with drive_unplug
From: |
Ryan Harper |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] v2 Fix Block Hotplug race with drive_unplug() |
Date: |
Thu, 21 Oct 2010 16:37:46 -0500 |
User-agent: |
Mutt/1.5.6+20040907i |
* Daniel P. Berrange <address@hidden> [2010-10-21 08:29]:
> On Tue, Oct 19, 2010 at 09:32:29AM -0500, Ryan Harper wrote:
> > Block hot unplug is racy since the guest is required to acknowlege the ACPI
> > unplug event; this may not happen synchronously with the device removal
> > command
> >
> > This series aims to close a gap where by mgmt applications that assume the
> > block resource has been removed without confirming that the guest has
> > acknowledged the removal may re-assign the underlying device to a second
> > guest
> > leading to data leakage.
> >
> > This series introduces a new montor command to decouple asynchornous device
> > removal from restricting guest access to a block device. We do this by
> > creating
> > a new monitor command drive_unplug which maps to a bdrv_unplug() command
> > which
> > does a qemu_aio_flush; bdrv_flush() and bdrv_close(). Once complete,
> > subsequent
> > IO is rejected from the device and the guest will get IO errors but
> > continue to
> > function.
> >
> > A subsequent device removal command can be issued to remove the device, to
> > which
> > the guest may or maynot respond, but as long as the unplugged bit is set,
> > no IO
> > will be sumbitted.
>
> The name 'drive_unplug' suggests to me that the drive object is
> not being deleted/free()d ? Is that correct understanding, and if
> so, what is responsible for finally free()ing the drive backend ?
It's technically the BlockDriverState Driver that we're closing. To
fully release the remaining resources, a device_del is required (which
of course requires guest participation with the current
interface).
Once QEMU issues the removal request, the guest responds and the piix4
acpi handler for pciej_write writes invokes qdev_free() on the target
device. qdev_free() on the pci device will make it's way to the qdev
exit handler registered for virtio-blk devices, virtio_blk_exit_pci().
virtio_blk_exit_pci() marks the drive structure for deletion. When qdev
calls the properties handler, it invokes free_drive() on the disk and
that calls blockdev_auto_del() which will do a bdrv_delete() which nukes
the remaining objects (the acutal BlockDriverState).
I think I got the whole path in there.
--
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
address@hidden
[Qemu-devel] [PATCH 1/2] v2 Add drive_get_by_id, Ryan Harper, 2010/10/19