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

["Michael S. Tsirkin" <mst@redhat.com>]

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.


> 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,
at this point it's easier to keep supporting it than
change all users ...


> 
>     --
>     David Gibson <dgibson@redhat.com>
>     Principal Software Engineer, Virtualization, Red Hat
>