Re: [PATCH v6 2/7] xen/arm: add pci-domain for disabled devices

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

Hi, Julien!

On 17.11.21 23:33, Julien Grall wrote:
> Hi Oleksandr,
>
> On 17/11/2021 06:56, Oleksandr Andrushchenko wrote:
>> Hi, Julien!
>>
>> On 16.11.21 20:48, Julien Grall wrote:
>>> Hi Oleksander,
>>>
>>> On 05/11/2021 06:33, Oleksandr Andrushchenko wrote:
>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>>>>
>>>> If a PCI host bridge device is present in the device tree, but is
>>>> disabled, then its PCI host bridge driver was not instantiated.
>>>> This results in the failure of the pci_get_host_bridge_segment()
>>>> and the following panic during Xen start:
>>>>
>>>> (XEN) Device tree generation failed (-22).
>>>> (XEN)
>>>> (XEN) ****************************************
>>>> (XEN) Panic on CPU 0:
>>>> (XEN) Could not set up DOM0 guest OS
>>>> (XEN) ****************************************
>>>>
>>>> Fix this by adding "linux,pci-domain" property for all device tree nodes
>>>> which have "pci" device type, so we know which segments will be used by
>>>> the guest for which bridges.
>>>>
>>>> Fixes: 4cfab4425d39 ("xen/arm: Add linux,pci-domain property for hwdom if 
>>>> not available.")
>>>>
>>>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>>>>
>>>> ---
>>>> New in v6
>>>> ---
>>>>    xen/arch/arm/domain_build.c        | 15 ++++++++++++++-
>>>>    xen/arch/arm/pci/pci-host-common.c |  2 +-
>>>>    xen/include/asm-arm/pci.h          |  8 ++++++++
>>>>    3 files changed, 23 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
>>>> index 491f5e2c316e..f7fcb1400c19 100644
>>>> --- a/xen/arch/arm/domain_build.c
>>>> +++ b/xen/arch/arm/domain_build.c
>>>> @@ -753,9 +753,22 @@ static int __init write_properties(struct domain *d, 
>>>> struct kernel_info *kinfo,
>>>>            {
>>>>                uint16_t segment;
>>>>    +            /*
>>>> +             * The node doesn't have "linux,pci-domain" property and it is
>>>> +             * possible that:
>>>> +             *  - Xen only has drivers for a part of the host bridges
>>>> +             *  - some host bridges are disabled
>>>> +             * Make sure we insert the correct "linux,pci-domain" property
>>>> +             * in any case, so we know which segments will be used
>>>> +             * by Linux for which bridges.
>>>
>>> The check above will check the node type is "pci". AFAICT, this would also 
>>> cover PCI devices. I am not aware of any issue to add "linux,pci-domain" 
>>> for them. However, this feels a bit odd.
>>>
>>>  From my understanding, a PCI device would always be described as a child 
>>> of the hostbridges. So I would rework the 'if' to also check if the parent 
>>> type is not "pci".
>>>
>> We may have "bridge -> bridge -> device" topology as well.
>
> Do you have an example of Device-Tree?
No, I don't have at hand, but I can imagine this can relatively easy be done 
with QEMU
Even if not, do you assume this topology can never happen?
>
>> So, I prefer to have the check as it is.
>
> I don't really like the idea to spuriously add "linux,pci-domain" to PCI DT 
> node. But if there are no other solution, then this should at least be 
> mentionned in the commit message and code.
I am fine with any solution here, I just want that to be defined and 
implemented.
Please let me know the final decision on this and how we proceed
>
> [...]
>
>> Yes, this sounds reasonable, I will add this change and print an error 
>> message so it is
>> easier to understand what Xen doesn't like (it took me a while to debug and 
>> understand
>> why I have "(XEN) Device tree generation failed (-22).")
>
>
> Sounds good to me. The current error is indeed confusing.
>
> Cheers,
>
Thank you,
Oleksandr