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

[Laine Stump <laine@redhat.com>]

On 11/03/2016 07:08 AM, Marcel Apfelbaum wrote:
> On 11/02/2016 06:01 PM, Laine Stump wrote:
>> On 11/02/2016 11:16 AM, 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
>>> and not only for hot-plug, without loosing any functionality.
>>> Older machines will have shpc enabled for compatibility reasons.
>>
>> From libvirt's POV, changing qemu's default for shpc in qemu only
>> serves to confuse; since we have no way to discover what is the
>> default setting (especially problematic since it is different for
>> different machinetype versions), we now either need to keep a table of
>> what the setting is for various machinetypes, or we need to just
>> always set it whether it's on or off.
>>
>> So for us, the simplest thing is for the default to remain the same
>> (since for so long we haven't set this option as we didn't even know
>> what it did, or indeed even that it existed), and for libvirt
>> to learn about the option, make its default in the config files "on",
>> but begin setting it to "off" in all newly defined machines.
>>
>
> Hi Laine,
>
> Please see my reply to Michael regarding the "why", anyway, can't
> libvirt deal with it?
> Start use the shpc parameter no matter what QEMU does by default while
> keeping it 'on' for machines < 2.8.
> (this is only a suggestion, of course...)


We greatly dislike coding in behavior changes to libvirt based on 
machinetype/version or qemu version since a version number is a very 
broad and inaccurate sword. The best behavior changes are those that can 
be discovered by querying something specific to that behavior, which 
can't be done in this case, as there is no way to query *anything* 
specific to a machinetype without instantiating a virtual machine of 
that machinetype (if it's even queryable then, which is anyway 
irrelevant to libvirt, since our queries to qemu are done with -machine 
none).

libvirt can of course deal with such a change in default behavior by 
qemu, but the way it will deal with it is by removing all assumptions 
about the default of shpc from the code, and replacing those assumptions 
with explicit setting of the option in the xml and on the qemu command 
line all the time.

In the end, libvirt wouldn't gain anything from this change in qemu's 
default behavior - it would be more work than just adding a setting for 
shpc to libvirt and having *libvirt* change its default setting (which 
has the same ultimate results). Of course that's a relatively selfish 
(libvirt-centric) view, and I suppose direct users of the qemu 
commandline might get some benefit from changing the qemu default, but 
anyone concerned enough about the exact commandline contents to be 
running qemu directly would probably also not have a problem adding an 
option to the commandline if they really wanted a 1/32 increase in the 
number of available slots.


>
>
> Thanks,
> Marcel
>
>> (The change to the default for virtio devices' disable-legacy
>> depending on where the device was connected could also have been
>> problematic, except that the change in default value of that has no
>> direct consequences on the rest of the configuration - libvirt will
>> still use the controllers in exactly the same way, and the same
>> commandlines for the same machinetype/versions will still result in
>> identical behavior (so it's not possible for libvirt to know whether
>> or not IO is enabled for the virtio device, nor to know what is the
>> device's . In the case of pci-bridge's shpc though, the whole
>> point of disabling shpc is to make another slot available for libvirt
>> to use for a device, so presumably the config *beyond the pci-bridge
>> device* needs to change as a result of this change in default
>> setting, so libvirt needs to know the setting and behave differently.)
>>
>>>
>>> Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
>>> ---
>>>  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 \
>>>
>>
>