Re: [Qemu-devel] [PATCH v3 5/5] docs: update documentation considering PCIE-PCI bridge

[Laszlo Ersek <lersek@redhat.com>]

On 08/02/17 15:47, Michael S. Tsirkin wrote:
> On Wed, Aug 02, 2017 at 12:23:46AM +0200, Laszlo Ersek wrote:
>> On 08/01/17 23:39, Michael S. Tsirkin wrote:
>>> On Wed, Aug 02, 2017 at 12:33:12AM +0300, Alexander Bezzubikov wrote:
>>>> 2017-08-01 23:31 GMT+03:00 Laszlo Ersek <lersek@redhat.com>:
>>>>> (Whenever my comments conflict with Michael's or Marcel's, I defer to them.)
>>>>>
>>>>> On 07/29/17 01:37, Aleksandr Bezzubikov wrote:
>>>>>> Signed-off-by: Aleksandr Bezzubikov <zuban32s@gmail.com>
>>>>>> ---
>>>>>>  docs/pcie.txt            |  46 ++++++++++--------
>>>>>>  docs/pcie_pci_bridge.txt | 121 +++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>  2 files changed, 147 insertions(+), 20 deletions(-)
>>>>>>  create mode 100644 docs/pcie_pci_bridge.txt
>>>>>>
>>>>>> diff --git a/docs/pcie.txt b/docs/pcie.txt
>>>>>> index 5bada24..338b50e 100644
>>>>>> --- a/docs/pcie.txt
>>>>>> +++ b/docs/pcie.txt
>>>>>> @@ -46,7 +46,7 @@ Place only the following kinds of devices directly on the Root Complex:
>>>>>>      (2) PCI Express Root Ports (ioh3420), for starting exclusively PCI Express
>>>>>>          hierarchies.
>>>>>>
>>>>>> -    (3) DMI-PCI Bridges (i82801b11-bridge), for starting legacy PCI
>>>>>> +    (3) PCIE-PCI Bridge (pcie-pci-bridge), for starting legacy PCI
>>>>>>          hierarchies.
>>>>>>
>>>>>>      (4) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root Buses
>>>>>
>>>>> When reviewing previous patches modifying / adding this file, I
>>>>> requested that we spell out "PCI Express" every single time. I'd like to
>>>>> see the same in this patch, if possible.
>>>>
>>>> OK, I didn't know it.
>>>>
>>>>>
>>>>>> @@ -55,18 +55,18 @@ Place only the following kinds of devices directly on the Root Complex:
>>>>>>     pcie.0 bus
>>>>>>     ----------------------------------------------------------------------------
>>>>>>          |                |                    |                  |
>>>>>> -   -----------   ------------------   ------------------   --------------
>>>>>> -   | PCI Dev |   | PCIe Root Port |   | DMI-PCI Bridge |   |  pxb-pcie  |
>>>>>> -   -----------   ------------------   ------------------   --------------
>>>>>> +   -----------   ------------------   -------------------   --------------
>>>>>> +   | PCI Dev |   | PCIe Root Port |   | PCIE-PCI Bridge |   |  pxb-pcie  |
>>>>>> +   -----------   ------------------   -------------------   --------------
>>>>>>
>>>>>>  2.1.1 To plug a device into pcie.0 as a Root Complex Integrated Endpoint use:
>>>>>>            -device <dev>[,bus=pcie.0]
>>>>>>  2.1.2 To expose a new PCI Express Root Bus use:
>>>>>>            -device pxb-pcie,id=pcie.1,bus_nr=x[,numa_node=y][,addr=z]
>>>>>> -      Only PCI Express Root Ports and DMI-PCI bridges can be connected
>>>>>> +      Only PCI Express Root Ports, PCIE-PCI bridges and DMI-PCI bridges can be connected
>>>>>
>>>>> It would be nice if we could keep the flowing text wrapped to 80 chars.
>>>>>
>>>>> Also, here you add the "PCI Express-PCI" bridge to the list of allowed
>>>>> controllers (and you keep DMI-PCI as permitted), but above DMI was
>>>>> replaced. I think these should be made consistent -- we should make up
>>>>> our minds if we continue to recommend the DMI-PCI bridge or not. If not,
>>>>> then we should eradicate all traces of it. If we want to keep it at
>>>>> least for compatibility, then it should remain as fully documented as it
>>>>> is now.
>>>>
>>>> Now I'm beginning to think that we shouldn't keep the DMI-PCI bridge
>>>> even for compatibility and may want to use a new PCIE-PCI bridge
>>>> everywhere (of course, except some cases when users are
>>>> sure they need exactly DMI-PCI bridge for some reason)
>>>
>>> Can dmi-pci support shpc? why doesn't it? For compatibility?
>>
>> I don't know why, but the fact that it doesn't is the reason libvirt
>> settled on auto-creating a dmi-pci bridge and a pci-pci bridge under
>> that for Q35. The reasoning was (IIRC Laine's words correctly) that the
>> dmi-pci bridge cannot receive hotplugged devices, while the pci-pci
>> bridge cannot be connected to the root complex. So both were needed.
>>
>> Thanks
>> Laszlo
> 
> OK. Is it true that dmi-pci + pci-pci under it will allow hotplug
> on Q35 if we just flip the bit in _OSC?

Marcel, what say you?... :)