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Re: [Qemu-devel] [PATCH 2/2] v2 Fix Block Hotplug race with drive_unplug

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH 2/2] v2 Fix Block Hotplug race with drive_unplug()
Date: Fri, 22 Oct 2010 12:31:41 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7

Am 21.10.2010 23:37, schrieb Ryan Harper:
> * 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).

So is this basically what blockdev_del is supposed to become one day?

Copying Markus to have a look at this. I'm sure he has some thoughts on
it as he was planning to implement blockdev_add/del.


> 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.

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