Re: [PATCH] pci: Refuse to hotplug PCI Devices when the Guest OS is not ready

[Peter Krempa <pkrempa@redhat.com>]

On Fri, Oct 23, 2020 at 19:27:55 +0200, Igor Mammedov wrote:
> On Fri, 23 Oct 2020 11:54:40 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Fri, Oct 23, 2020 at 09:47:14AM +0300, Marcel Apfelbaum wrote:
> > > Hi David,
> > > 
> > > On Fri, Oct 23, 2020 at 6:49 AM David Gibson <dgibson@redhat.com> wrote:
> > > 
> > >     On Thu, 22 Oct 2020 11:01:04 -0400
> > >     "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > >   
> > >     > On Thu, Oct 22, 2020 at 05:50:51PM +0300, Marcel Apfelbaum wrote:
> > >     >  [...] 
> > >     >
> > >     > Right. After detecting just failing unconditionally it a bit too
> > >     > simplistic IMHO.  
> > > 
> > >     There's also another factor here, which I thought I'd mentioned
> > >     already, but looks like I didn't: I think we're still missing some
> > >     details in what's going on.
> > > 
> > >     The premise for this patch is that plugging while the indicator is in
> > >     transition state is allowed to fail in any way on the guest side.  I
> > >     don't think that's a reasonable interpretation, because it's unworkable
> > >     for physical hotplug.  If the indicator starts blinking while you're in
> > >     the middle of shoving a card in, you'd be in trouble.
> > > 
> > >     So, what I'm assuming here is that while "don't plug while blinking" is
> > >     the instruction for the operator to obey as best they can, on the guest
> > >     side the rule has to be "start blinking, wait a while and by the time
> > >     you leave blinking state again, you can be confident any plugs or
> > >     unplugs have completed".  Obviously still racy in the strict computer
> > >     science sense, but about the best you can do with slow humans in the
> > >     mix.
> > > 
> > >     So, qemu should of course endeavour to follow that rule as though it
> > >     was a human operator on a physical machine and not plug when the
> > >     indicator is blinking.  *But* the qemu plug will in practice be fast
> > >     enough that if we're hitting real problems here, it suggests the guest
> > >     is still doing something wrong.
> > > 
> > > 
> > > I personally think there is a little bit of over-engineering here.
> > > Let's start with the spec:
> > > 
> > >     Power Indicator Blinking
> > >     A blinking Power Indicator indicates that the slot is powering up or
> > > powering down and that
> > >     insertion or removal of the adapter is not permitted.
> > > 
> > > What exactly is an interpretation here?
> > > As you stated, the races are theoretical, the whole point of the indicator
> > > is to let the operator know he can't plug the device just yet.
> > > 
> > > I understand it would be more user friendly if the QEMU would wait internally
> > > for the
> > > blinking to end, but the whole point of the indicator is to let the operator 
> > > (human or machine)
> > > know they can't plug the device at a specific time.
> > > Should QEMU take the responsibility of the operator? Is it even correct?
> > > 
> > > Even if we would want such a feature, how is it related to this patch?
> > > The patch simply refuses to start a hotplug operation when it knows it will not
> > > succeed. 
> > >  
> > > Another way that would make sense to me would be  is a new QEMU interface other
> > > than
> > > "add_device", let's say "adding_device_allowed", that would return true if the
> > > hotplug is allowed
> > > at this point of time. (I am aware of the theoretical races)   
> > 
> > Rather than adding_device_allowed, something like "query slot"
> > might be helpful for debugging. That would help user figure out
> > e.g. why isn't device visible without any races.
> 
> Would be new command useful tough? What we end up is broken guest
> (if I read commit message right) and a user who has no idea if 
> device_add was successful or not.
> So what user should do in this case
>   - wait till it explodes?
>   - can user remove it or it would be stuck there forever?
>   - poll slot before hotplug, manually?
> 
> (if this is the case then failing device_add cleanly doesn't sound bad,
> it looks similar to another error we have "/* Check if hot-plug is disabled on the slot */"
> in pcie_cap_slot_pre_plug_cb)
> 
> CCing libvirt, as it concerns not only QEMU.

The only reason a separate command might make sense is if libvirt would
want to provide a specific error to the user/upper management layer that
the operation failed due to a transient failure (so that it can be
retried later).

We don't really want to go to a policy decision of how long to wait in
such case, so unless qemu waits libvirt will plainly want to report an
error.

That said IMO 'device_add' should still fail if it's certain that the
device won't be plugged in. This will fix any client which is currently
in use. Adding a separate command is worth only for pre-checking for
saner error handling.

> > > The above will at least mimic the mechanics of the pyhs world.  The operator
> > > looks at the indicator,
> > > the management software checks if adding the device is allowed.
> > > Since it is a corner case I would prefer the device_add to fail rather than
> > > introducing a new interface,
> > > but that's just me.
> > > 
> > > Thanks,
> > > Marcel
> > >   
> > 
> > I think we want QEMU management interface to be reasonably
> > abstract and agnostic if possible. Pushing knowledge of hardware
> > detail to management will just lead to pain IMHO.
> > We supported device_add which practically never fails for years,
> 
> For CPUs and RAM, device_add can fail so maybe management is also
> prepared to handle errors on PCI hotplug path.

While it was me who implemented device_add for cpu/memory I don't
remmeber any more whether we are really prepared for it.

We certainly want to do it if there's a possibility to do it.