Re: [PATCH] kvm: Check irqchip mode before assign irqfd

[Peter Xu <peterx@redhat.com>]

On Sun, May 05, 2019 at 05:20:22PM +0800, Peter Xu wrote:
> On Sun, May 05, 2019 at 04:56:42PM +0800, Peter Xu wrote:
> > When assigning kvm irqfd we didn't check the irqchip mode but we allow
> > KVM_IRQFD to succeed with all the irqchip modes.  However it does not
> > make much sense to create irqfd even without the kernel chips.  Let's
> > provide a arch-dependent helper to check whether a specific irqfd is
> > allowed by the arch.  At least for x86, it should make sense to check:
> > 
> > - when irqchip mode is NONE, all irqfds should be disallowed, and,
> > 
> > - when irqchip mode is SPLIT, irqfds that are with resamplefd should
> >   be disallowed.
> > 
> > For either of the case, previously we'll silently ignore the irq or
> > the irq ack event if the irqchip mode is incorrect.  However that can
> > cause misterious guest behaviors and it can be hard to triage.  Let's
> > fail KVM_IRQFD even earlier to detect these incorrect configurations.
> > 
> > CC: Paolo Bonzini <pbonzini@redhat.com>
> > CC: Radim Krčmář <rkrcmar@redhat.com>
> > CC: Alex Williamson <alex.williamson@redhat.com>
> > CC: Eduardo Habkost <ehabkost@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> Note: haven't tested, but IIUC QEMU's vfio will naturally fall back to
> no-irqfd mode (actually virtio seems to also have this but virtio
> should not be affected after all) if the KVM_IRQFD ioctl failed so I
> feel like this patch could also at least fix the broken guests
> reported besides any future fixes from QEMU side on the issue:
> 
> https://bugs.launchpad.net/qemu/+bug/1826422

Ping - Paolo, should patch still worth to have before the complete fix
between split irqchip & resamplefds?

Thanks,

-- 
Peter Xu