Re: [Qemu-devel] [PATCH] hw/pci: disable pci-bridge's shpc by default

[Andrew Jones <drjones@redhat.com>]

On Sat, Nov 05, 2016 at 06:46:34PM +0200, Marcel Apfelbaum wrote:
> On 11/03/2016 09:40 PM, Michael S. Tsirkin wrote:
> > On Thu, Nov 03, 2016 at 01:05:44PM +0200, Marcel Apfelbaum wrote:
> > > On 11/03/2016 06:18 AM, Michael S. Tsirkin wrote:
> > > > On Wed, Nov 02, 2016 at 05:16:42PM +0200, Marcel Apfelbaum wrote:
> > > > > The shpc component is optional while  ACPI hotplug is used
> > > > > for hot-plugging PCI devices into a PCI-PCI bridge.
> > > > > Disabling the shpc by default will make slot 0 usable at boot time
> > > 
> > > Hi Michael
> > > 
> > > > 
> > > > at the cost of breaking all hotplug for all non-acpi users.
> > > > 
> > > 
> > > Do we have a non-acpi user that is able to use the shpc component as-is today?
> > 
> > power and some arm systems I guess?
> > 
> 
> Adding Andrew , maybe he can give us an answer.

Not really :-) My lack of PCI knowledge makes that difficult. I'd be happy
to help with an experiment though. Can you give me command line arguments,
qmp commands, etc. that I should use to try it out? I imagine I should
just boot an ARM guest using DT (instead of ACPI) and then attempt to
hotplug a PCI device. I'm not sure, however, what, if any, special
configuration I need in order to ensure I'm testing what you're
interested in.

Thanks,
drew


> 
> Anybody else can help answering this?
> 
> > > I remember we need to even tweak QEMU before it can be used, but I might be wrong.
> > > 
> > > And we don't touch the current machines < 2.8 .
> > > 
> > > > > and not only for hot-plug, without loosing any functionality.
> > > > > Older machines will have shpc enabled for compatibility reasons.
> > > > > 
> > > > > Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
> > > > 
> > > > Is an extra slot such a big deal? You can always add more bridges ...
> > > > 
> > > 
> > > It is not only about the slot itself, but more about the usage model.
> > > The PCIe Upstream ports/DMI-PCI devices are also pci-bridges,
> > > but for them slot 0 is allowed.
> > 
> > The reason is that these devices are not themselves
> > hotpluggable. Isn't there a flag that allows adding
> > a non hotpluggable device? Allowing these would be one solution.
> > 
> > > And what about the hotplug? Slot 0 is not usable at boot, but then is
> > > usable again (for ACPI users) making people wondering:
> > >  https://bugzilla.redhat.com/show_bug.cgi?id=1175113
> > 
> > Let's just disallow that then for consistency?
> > 
> 
> I suppose we can do that... not sure if it worth it.
> 
> Thanks,
> Marcel
> 
> > 
> > > My point is - can shpc be used as-is today? Even so, I suspect there are much (much)
> > > less users using SHPC than ACPI based hotplug. If this is the case, why bother the
> > > majority of the users? And for the shpc users, they can keep the prev machines
> > > or change the command line, I think changes like this happens over the time.
> > > 
> > > Adding Markus for his opinion on command line changes.
> > > 
> > > Thanks,
> > > Marcel
> > > > > ---
> > > > >  hw/pci-bridge/pci_bridge_dev.c | 2 +-
> > > > >  include/hw/compat.h            | 4 ++++
> > > > >  2 files changed, 5 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/hw/pci-bridge/pci_bridge_dev.c b/hw/pci-bridge/pci_bridge_dev.c
> > > > > index 5dbd933..647ad80 100644
> > > > > --- a/hw/pci-bridge/pci_bridge_dev.c
> > > > > +++ b/hw/pci-bridge/pci_bridge_dev.c
> > > > > @@ -163,7 +163,7 @@ static Property pci_bridge_dev_properties[] = {
> > > > >      DEFINE_PROP_ON_OFF_AUTO(PCI_BRIDGE_DEV_PROP_MSI, PCIBridgeDev, msi,
> > > > >                              ON_OFF_AUTO_AUTO),
> > > > >      DEFINE_PROP_BIT(PCI_BRIDGE_DEV_PROP_SHPC, PCIBridgeDev, flags,
> > > > > -                    PCI_BRIDGE_DEV_F_SHPC_REQ, true),
> > > > > +                    PCI_BRIDGE_DEV_F_SHPC_REQ, false),
> > > > >      DEFINE_PROP_END_OF_LIST(),
> > > > >  };
> > > > > 
> > > > > diff --git a/include/hw/compat.h b/include/hw/compat.h
> > > > > index 0f06e11..388b7ec 100644
> > > > > --- a/include/hw/compat.h
> > > > > +++ b/include/hw/compat.h
> > > > > @@ -18,6 +18,10 @@
> > > > >          .driver   = "intel-iommu",\
> > > > >          .property = "x-buggy-eim",\
> > > > >          .value    = "true",\
> > > > > +    },{\
> > > > > +        .driver   = "pci-bridge",\
> > > > > +        .property = "shpc",\
> > > > > +        .value    = "on",\
> > > > >      },
> > > > > 
> > > > >  #define HW_COMPAT_2_6 \
> > > > > --
> > > > > 2.5.5
> 
>