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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: Tue, 27 Oct 2020 07:30:51 -0400 
On Fri, Oct 23, 2020 at 09:26:48AM +0300, Marcel Apfelbaum wrote:
> Hi Michael,
> 
> On Thu, Oct 22, 2020 at 6:01 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>     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.
> 
> 
> 
> Simplistic does not mean wrong or incorrect.
> I fail to see why it is not enough.

The failure patch requires management to retry later.
A more elaborate scheme will fix the bug without need for management
changes.


> What QEMU can do better? Wait an unbounded time for the blinking to finish?
> What if we have a buggy guest with a kernel stuck in blinking?

Then it won't see the new device ever but does it even matter? It's
stuck ... I'd ack adding a query command to see what is going
on with the device. Can be generic, implementable on top of ACPI too.

> Is QEMU's responsibility to emulate the operator itself? Because the operator
> is the one who is supposed to wait.

I think these details are immaterial for users. They don't read pci
spec.

> 
> Thanks,
> Marcel
> 
> [...] 
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