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

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

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?

> 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?


> 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