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Re: [libvirt] [PATCH] qemu: call drive_unplug in DetachPciDiskDevice

On Thu, Oct 21, 2010 at 11:53:29AM -0500, Ryan Harper wrote:
> * Daniel P. Berrange <berrange redhat com> [2010-10-21 11:46]:
> > On Thu, Oct 21, 2010 at 08:50:35AM -0500, Ryan Harper wrote:
> > > Currently libvirt doesn't confirm whether the guest has responded to the
> > > disk removal request.  In some cases this can leave the guest with
> > > continued access to the device while the mgmt layer believes that it has
> > > been removed.  With a recent qemu monitor command[1] we can
> > > deterministically revoke a guests access to the disk (on the QEMU side)
> > > to ensure no futher access is permitted.
> > > 
> > > This patch adds support for the drive_unplug() command and introduces it
> > > in the disk removal paths.  There is some discussion to be had about how
> > > to handle the case where the guest is running in a QEMU without this
> > > command (and the fact that we currently don't have a way of detecting
> > > what monitor commands are available).
> > 
> > Basically we try to run the command and then catch the failure.
> > 
> > For QMP, we can check for a error with a class of 'CommandNotFound',
> > 
> > For HMP, QEMU will print 'unknown command' in the reply.
> > 
> > Neither is ideal, since neither is a guarenteed part of the monitor
> > interface, but it is all we have to go on, and ensure other critical
> > errors will still be treated as fatal by libvirt.
> Yeah, this sorta already works without explictly catching the error
> (unless you want to display a different message at caller level than
> command not found).

This would cause the hot-unplug operation to completely terminate
though, whereas we want it to carry on, if the command is not
present. If we get a command-not-found, then we should syslog a 
warning, and then return '0' (ie success) to the caller of

Alternatively, return '+1' to the caller, and make the callers
check for '+1', syslog it and treat it as non-fatal. This is
closer to the approach we use for handling 'balloon device not
present' error when code runs the 'info balloon' method.

> > > My current implementation assumes that if you don't have a QEMU with
> > > this capability that we should fail the device removal.  This is a
> > > strong statement around hotplug that isn't consistent with previous
> > > releases so I'm open to other approachs, but given the potential data
> > > leakage problem hot-remove can lead to without drive_unplug, I think
> > > it's the right thing to do.
> > 
> > I don't think we can do this, since it obviously breaks every single
> > existing deployment out there. Users who have sVirt enabled will
> > have a level of protection from the data leakage, so I don't think
> > it is a severe problem. 
> Yeah, I was thinking we could re-work the logic to only completely fail
> in the case where the command wasn't found/successful *and* we don't
> have sVirt enabled.

I'm still wary of doing that because of the significant negative
impact on existing deployments out there, even in the case where
hotunplug would complete just fine. I think we'll just need to
syslog the potential issue and people can upgrade their QMEU if
they need the extra security

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