Re: [Qemu-devel] [PATCH v8 11/12] vfio: register aer resume notification handler for aer resume

[Alex Williamson <alex.williamson@redhat.com>]

On Sun, 10 Jul 2016 09:28:41 +0800
Zhou Jie <zhoujie2011@cn.fujitsu.com> wrote:

> Hi Alex,
> 
> > The variable clearly isn't visible to the user, so the user can know
> > whether the kernel supports this feature, but not whether the feature
> > is currently active.  Perhaps there's no way to avoid races completely,
> > but don't you expect that if we define that certain operations are
> > blocked after an error notification that a user may want some way to
> > poll for whether the block is active after a reset rather than simply
> > calling a blocked interface to probe for it?  
> Yes, I will use access blocked function, not the variable.

I don't understand what this means.
 
> > As we've discussed before, the AER notification needs to be relayed to
> > the user without delay, otherwise we only increase the gap where the
> > user might consume bogus data.  It also only seems reasonable to modify
> > the behavior of the interfaces (ie. blocking) if the user is notified,
> > which would be through the existing error notifier.  We can never
> > depend on a specific behavior from the user, we may be dealing with a
> > malicious user.
> >
> > We already disable interrupts in vfio_pci_disable() simply by calling
> > the ioctl function directly.  
> Sorry, I want to know where is vfio_pci_disable invoked.
> I can't find it in ioctl function.

You have the code, vfio_pci_disable() is invoked when the vfio device
file descriptor is released.  It's not in the ioctl, it calls the ioctl
as the user would to disable all interrupts on the device.

> > If we simply disable and re-enable interrupts as you propose,
> > how does the user deal with edge triggered
> > interrupts that may have occurred during that period?  Are they lost?
> > Should we instead leave the interrupts enabled but skip
> > eventfd_signal() in the interrupts handlers, queuing interrupts for
> > re-delivery after the device is resumed?  
> Yes, they will lost.

Is that acceptable?  This is part of the problem I have with silently
disabling interrupt delivery via the command register across reset.  It
seems more non-deterministic than properly disabling interrupts and
requiring the user to reinitialize them after error.
 
> > Or does it make more sense to
> > simply disable the interrupts as done in vfio_pci_disable() and define
> > that the user needs to re-establish interrupts before continuing after
> > an error event?  Thanks,  
> If user invoked the vfio_pci_disable by ioctl function.

I'm in no way suggesting that a user invoke vfio_pci_disable(), I'm just
trying to point out that vfio_pci_disable() already does a teardown of
interrupts, similar to what seems to be required here.

> Yes, user should re-establish interrupts before
> continuing after an error event.

So if we define that users should re-establish interrupts after an
error event, then what's the point of only doing command register
masking of the interrupts and requiring the user to both tear-down the
interrupts and re-establish them?  Thanks,

Alex