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

[David Gibson]

On Fri, 23 Oct 2020 19:27:55 +0200
Igor Mammedov <imammedo@redhat.com> wrote:

> On Fri, 23 Oct 2020 11:54:40 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
>  [...]  
>  [...]  
>  [...]  
>  [...]  
> > 
> > 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.
> 
>  [...]  
>  [...]  
> > 
> > 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.

There can be unarguable reasons for PCI hotplug to fail as well
(attempting to plug to a bus that can't support it for one).  The
difference here is that it's a failure that we expect to be transitory.

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

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