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Re: [Qemu-devel] [PATCH for-2.10 0/3] qdev/vfio: defer DEVICE_DEL to avo
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCH for-2.10 0/3] qdev/vfio: defer DEVICE_DEL to avoid races with libvirt |
Date: |
Wed, 9 Aug 2017 15:08:42 +1000 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
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" <address@hidden> 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 <address@hidden> 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.
--
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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