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

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jul 27, 2017 at 08:47:33AM -0600, Alex Williamson wrote:
> On Thu, 27 Jul 2017 20:53:48 +1000
> David Gibson <david@gibson.dropbear.id.au> 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.
> 
> It's not me you need to convince, it's GregKH[1].  That interpretation
> is that the user bind request is a mandate and we'll fall over
> ourselves to try to do as they ask.  I think the best I might be able
> to do is to kill the QEMU process to avoid compromising the kernel
> rather than killing the kernel after the isolation compromise has
> occurred.  Messing with driver binding is a privileged operation, and
> the kernel believes you get to keep all the pieces when it fails.
> Sorry.  Thanks,

Ah, my mistake.  Well, I guess I'll whinge at GregKH at some point.

Anyway, the basic point remains - I still think we should address this
in the kernel, but that's not going to happen soon.  So we're left
with addressing it in qemu and/or libvirt.  And as others have pointed
out, there are reasons to do that even if the kernel does get changed
to protect itself better here.

> 
> Alex
> 
> [1] https://lkml.org/lkml/2017/7/10/728
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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