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

From: Michael S. Tsirkin
Subject: Re: [PATCH] pci: Refuse to hotplug PCI Devices when the Guest OS is not ready
Date: Thu, 22 Oct 2020 11:01:04 -0400 
On Thu, Oct 22, 2020 at 05:50:51PM +0300, Marcel Apfelbaum wrote:
> 
> 
> On Thu, Oct 22, 2020 at 5:33 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>     On Thu, Oct 22, 2020 at 05:10:43PM +0300, Marcel Apfelbaum wrote:
>     >
>     >
>     > On Thu, Oct 22, 2020 at 5:01 PM Michael S. Tsirkin <mst@redhat.com>
>     wrote:
>     >
>     >     On Thu, Oct 22, 2020 at 04:55:10PM +0300, Marcel Apfelbaum wrote:
>     >     > Hi David, Michael,
>     >     >
>     >     > On Thu, Oct 22, 2020 at 3:56 PM David Gibson 
> <dgibson@redhat.com>
>     wrote:
>     >     >
>     >     >     On Thu, 22 Oct 2020 08:06:55 -0400
>     >     >     "Michael S. Tsirkin" <mst@redhat.com> wrote:
>     >     >
>     >     >     > On Thu, Oct 22, 2020 at 02:40:26PM +0300, Marcel 
> Apfelbaum
>     wrote:
>     >     >     > > From: Marcel Apfelbaum <marcel@redhat.com>
>     >     >     > >
>     >     >     > > During PCIe Root Port's transition from Power-Off to
>     Power-ON (or
>     >     >     vice-versa)
>     >     >     > > the "Slot Control Register" has the "Power Indicator
>     Control"
>     >     >     > > set to "Blinking" expressing a "power transition" 
> mode.
>     >     >     > >
>     >     >     > > Any hotplug operation during the "power transition" 
> mode is
>     not
>     >     >     permitted
>     >     >     > > or at least not expected by the Guest OS leading to 
> strange
>     >     failures.
>     >     >     > >
>     >     >     > > Detect and refuse hotplug operations in such case.
>     >     >     > >
>     >     >     > > Signed-off-by: Marcel Apfelbaum 
> <marcel.apfelbaum@gmail.com
>     >
>     >     >     > > ---
>     >     >     > >  hw/pci/pcie.c | 7 +++++++
>     >     >     > >  1 file changed, 7 insertions(+)
>     >     >     > >
>     >     >     > > diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c
>     >     >     > > index 5b48bae0f6..2fe5c1473f 100644
>     >     >     > > --- a/hw/pci/pcie.c
>     >     >     > > +++ b/hw/pci/pcie.c
>     >     >     > > @@ -410,6 +410,7 @@ void pcie_cap_slot_pre_plug_cb
>     (HotplugHandler
>     >     >     *hotplug_dev, DeviceState *dev,
>     >     >     > >      PCIDevice *hotplug_pdev = 
> PCI_DEVICE(hotplug_dev);
>     >     >     > >      uint8_t *exp_cap = hotplug_pdev->config +
>     hotplug_pdev->
>     >     >     exp.exp_cap;
>     >     >     > >      uint32_t sltcap = pci_get_word(exp_cap +
>     PCI_EXP_SLTCAP);
>     >     >     > > +    uint32_t sltctl = pci_get_word(exp_cap +
>     PCI_EXP_SLTCTL);
>     >     >     > > 
>     >     >     > >      /* Check if hot-plug is disabled on the 
> slot */
>     >     >     > >      if (dev->hotplugged && (sltcap & 
> PCI_EXP_SLTCAP_HPC) =
>     = 0) {
>     >     >     > > @@ -418,6 +419,12 @@ void pcie_cap_slot_pre_plug_cb
>     >     (HotplugHandler
>     >     >     *hotplug_dev, DeviceState *dev,
>     >     >     > >          return;
>     >     >     > >      }
>     >     >     > > 
>     >     >     > > +    if ((sltctl & PCI_EXP_SLTCTL_PIC) ==
>     >     PCI_EXP_SLTCTL_PWR_IND_BLINK)
>     >     >     {
>     >     >     > > +        error_setg(errp, "Hot-plug failed: %s 
> is in Power
>     >     Transition",
>     >     >     > > +                   
> DEVICE(hotplug_pdev)->id);
>     >     >     > > +        return;
>     >     >     > > +    }
>     >     >     > > +
>     >     >     > >      
> pcie_cap_slot_plug_common(PCI_DEVICE(hotplug_dev),
>     dev,
>     >     errp);
>     >     >     > >  } 
>     >     >     >
>     >     >     > Probably the only way to handle for existing machine 
> types.
>     >     >
>     >     >
>     >     > I agree
>     >     >  
>     >     >
>     >     >     > For new ones, can't we queue it in host memory 
> somewhere?
>     >     >
>     >     >
>     >     >
>     >     > I am not sure I understand what will be the flow.
>     >     >   - The user asks for a hotplug operation.
>     >     >   -  QEMU deferred operation.
>     >     > After that the operation may still fail, how would the user 
> know if
>     the
>     >     > operation
>     >     > succeeded or not?
>     >
>     >
>     >     How can it fail? It's just a button press ...
>     >
>     >
>     >
>     > Currently we have "Hotplug unsupported."
>     > With this change we have "Guest/System not ready"
> 
> 
>     Hotplug unsupported is not an error that can trigger with
>     a well behaved management such as libvirt.
> 
> 
>     >  
>     >
>     >     >  
>     >     >
>     >     >     I'm not actually convinced we can't do that even for 
> existing
>     machine
>     >     >     types. 
>     >     >
>     >     >
>     >     > Is a Guest visible change, I don't think we can do it.
>     >     >  
>     >     >
>     >     >     So I'm a bit hesitant to suggest going ahead with this 
> without
>     >     >     looking a bit closer at whether we can implement a
>     wait-for-ready in
>     >     >     qemu, rather than forcing every user of qemu (human or 
> machine)
>     to do
>     >     >     so.
>     >     >
>     >     >
>     >     > While I agree it is a pain from the usability point of view,
>     hotplug
>     >     operations
>     >     > are allowed to fail. This is not more than a corner case, 
> ensuring
>     the
>     >     right
>     >     > response (gracefully erroring out) may be enough.
>     >     >
>     >     > Thanks,
>     >     > Marcel
>     >     >
>     >
>     >
>     >     I don't think they ever failed in the past so management is 
> unlikely
>     >     to handle the failure by retrying ...
>     >
>     >
>     > That would require some management handling, yes.
>     > But even without a "retry", failing is better than strange OS behavior.
>     >
>     > Trying a better alternative like deferring the operation for new 
> machines
>     > would make sense, however is out of the scope of this patch
> 
>     Expand the scope please. The scope should be "solve a problem xx" not
>     "solve a problem xx by doing abc".
> 
> 
> 
> The scope is detecting a hotplug error early instead
> passing to the Guest OS a hotplug operation that we know it will fail.
> 

Right. After detecting just failing unconditionally it a bit too
simplistic IMHO.

> 
>     > that simply
>     > detects the error leaving us in a slightly better state than today.
>     >
>     > Thanks,
>     > Marcel
> 
>     Not applying a patch is the only tool we maintainers have to influence
>     people to solve the problem fully. 
> 
>     That's why I'm not inclined to apply
>     "slightly better" patches generally.
> 
> 
> 
> The patch is a proposal following some offline discussions on this matter.
> I personally see the value of it versus what we have today.
> 
> Thanks,
> Marcel
> 
> 
>     >
>     >
>     >     >
>     >     >
>     >     >
>     >     >     --
>     >     >     David Gibson <dgibson@redhat.com>
>     >     >     Principal Software Engineer, Virtualization, Red Hat
>     >     >
>     >
>     >
> 
>
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