On Wed, 9 Aug 2017 15:08:42 +1000 David Gibson <da...@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" <berra...@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 <mdr...@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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