Re: [Qemu-devel] [PATCH for-2.10 0/3] qdev/vfio: defer DEVICE_DEL to avoid races with libvirt

[Greg Kurz <groug@kaod.org>]

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On Wed, 9 Aug 2017 15:08:42 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Aug 08, 2017 at 01:40:08PM -0600, Alex Williamson wrote:
> > On Thu, 27 Jul 2017 12:50:42 +0100
> > "Daniel P. Berrange" <berrange@redhat.com> wrote:
> >   
> > > On Thu, Jul 27, 2017 at 08:53:48PM +1000, David Gibson wrote:  
> > > > On Thu, Jul 27, 2017 at 10:11:48AM +0100, Peter Maydell wrote:    
> > > > > On 27 July 2017 at 02:30, Michael Roth <mdroth@linux.vnet.ibm.com> wrote:    
> > > > > > In particular, Mellanox CX4 adapters on PowerNV hosts might not be fully
> > > > > > quiesced by vfio-pci's finalize() routine until up to 6s after the
> > > > > > DEVICE_DELETED was emitted, leading to detach-device on the libvirt side pretty
> > > > > > much always crashing the host.    
> > > > > 
> > > > > My initial naive thought is that if the host kernel can crash then
> > > > > this is a host kernel bug... shouldn't the host kernel refuse
> > > > > the subsequent libvirt rebind if it would cause a crash ?    
> > > > 
> > > > I think so too, but I haven't been able to convince Alex.  Nor
> > > > find time to fix it in the kernel myself.    
> > > 
> > > I think we need to fix both the QEMU premature sending of DEVICE_DELETED
> > > and the kernel bug that allowed the crash.  
> > 
> > 
> > Where do we stand on this for v2.10?  I'd like to see it get in.  There
> > may be things to fix in the kernel, some of them may already be fixed
> > in the latest development kernel, but ultimately the kernel considers
> > driver binding to be a trusted operation and if userspace doesn't
> > understand all the dependencies, they shouldn't be doing it.  In this
> > case libvirt is using the DEVICE_DELETED signal with the assumption
> > that the device has been fully released by QEMU, which is of course not
> > accurate (libvirt could test this, but chooses not to).  libvirt
> > therefore begins trying to unbind a device that is still in use, we try
> > to handle it, but see official kernel stance that userspace is
> > responsible for understanding device dependencies, so we can only do so
> > much.
> > 
> > IMO, the next step along those lines would be that libvirt needs to
> > understand that even once a device is fully released from QEMU, it's
> > not necessarily safe to re-bind the device to a host driver.  If the
> > device is a member of a group where other devices are still in use by
> > userspace, this will violate user/host device isolation and the kernel
> > will crash to protect itself.  At best I may be able to improve this to
> > killing the userspace process making use of the conflicting device, but
> > the kernel view is that userspace (libvirt) has mandated to bind the
> > device to the host driver and we must make it so, the user is
> > responsible for the consequences.  Thanks,  
> 
> Merging it for 2.10 seems like a good idea to me to, but it's not
> really my area of expertise, and therefore not my call.
> 

It looks like there was some kind of consensus on this series, but it
didn't make it for 2.10. Is there something more to discuss ?

Cheers,

--
Greg

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