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Re: [qemu-s390x] [RFC PATCH 1/3] hw/Kconfig: PCI bus implies PCI_DEVICES

From: Thomas Huth
Subject: Re: [qemu-s390x] [RFC PATCH 1/3] hw/Kconfig: PCI bus implies PCI_DEVICES
Date: Mon, 15 Jul 2019 15:19:17 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

On 15/07/2019 13.09, Cornelia Huck wrote:
> On Mon, 15 Jul 2019 13:04:28 +0200
> Philippe Mathieu-Daudé <address@hidden> wrote:
>> On 7/15/19 12:56 PM, Cornelia Huck wrote:
>>> On Mon, 15 Jul 2019 12:48:55 +0200
>>> Thomas Huth <address@hidden> wrote:
>>>> On 15/07/2019 12.19, Peter Maydell wrote:  
>>>>> On Mon, 15 Jul 2019 at 11:15, Thomas Huth <address@hidden> wrote:    
>>>>>> On 15/07/2019 11.55, Philippe Mathieu-Daudé wrote:    
>>>>>>> If a controller device provides a PCI bus, we can plug any PCI
>>>>>>> daughter card on it.
>>>>>>> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
>>>>>>> ---    
>>>>>>> diff --git a/hw/pci/Kconfig b/hw/pci/Kconfig
>>>>>>> index 77f8b005ff..0f7267db35 100644
>>>>>>> --- a/hw/pci/Kconfig
>>>>>>> +++ b/hw/pci/Kconfig
>>>>>>> @@ -1,5 +1,6 @@
>>>>>>>  config PCI
>>>>>>>      bool
>>>>>>> +    imply PCI_DEVICES    
>>>>>> No, please don't change this. This was done on purpose, since almost all
>>>>>> PCI_DEVICES do not work on s390x (so s390x does *not* imply 
>>>>>> PCI_DEVICES).    
>>>>> But that means that every board that provides PCI has to have an
>>>>> "imply PCI_DEVICES" line, which is pretty clunky just to work
>>>>> around an s390x limitation.
>>>>> Is there some way in the Kconfig syntax for s390x to say
>>>>> "no PCI_DEVICES" so we can have the corner-case be handled
>>>>> by the s390x Kconfig in one place rather than in 20 places
>>>>> affecting everywhere except s390x?    
>>>> IIRC the problem on s390x are the legacy IRQs. s390x has only MSIs. So I
>>>> guess the correct way to fix this would be to introduce some
>>>> PCI_LEGACY_IRQ switch and let all old devices that do not work with MSI
>>>> depend on it.  
>>> s/MSI/MSI-X/, IIRC. Not sure how far 'legacy' would stretch.  
>> Maybe we can have something like PCI_LEGACY_DEVICES and PCI_MSI_DEVICES?
>> So if s390x only selects PCI_LEGACY (not PCI_MSI) bus, then it only get
>> legacy devices?
> Wrong way around? We need MSI-X for s390x, not plain MSI or
> 'legacy' (whatever that is).

With "legacy" I meant the old level-triggered interrupts from the early
PCI (non-express) days. Sorry for being imprecise here.

So maybe we need two new switches, PCI_CLASSIC (or so) and PCI_MSIX, and
then the PCI devices should be marked with "default y if PCI_CLASSIC" if
they do not have MSIX support, and with "default y if PCI_MSIX" if they
have MSI-X support?



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