Re: [PATCH v1 09/14] xen/arm: Add cmdline boot option "pci=on"

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.08.2021 16:34, Julien Grall wrote:
> On 20/08/2021 13:19, Rahul Singh wrote:
>>> On 19 Aug 2021, at 1:31 pm, Julien Grall <julien@xxxxxxx> wrote:
>>> On 19/08/2021 13:02, Rahul Singh wrote:
>>>> Add cmdline boot option "pci=on" to enable/disable the PCI init during
>>>> boot.
>>>
>>> I read this as "PCI" will be either disabled/enabled for the platform. 
>>> Whereas, I think it will be used to decide whether Xen discover PCI and PCI 
>>> passthrough is supported or not.
>>
>> Yes. I will modify the option to "pci-passthrough== <boolean>"
>>>
>>> Can you also clarify why a user would want to select "pci=off"?
>>
>> As pci-passthrough support emulate the PCI devices for DOM0 also, I thought 
>> if someone want to
>> boot the DOM0 without emulating the PCI device in XEN and wants to have 
>> direct access to device.
> 
> Dom0 will always have direct access to the PCI device. The only 
> difference is whether the access to the hostbridge and config space will 
> be trapped by Xen. I expect the both to mainly happen during boot and 
> therefore the overhead will be limited.
> 
>>
>> I am ok to drop this patch if you feel adding the option is not required at 
>> all.
> One of the reason I could see this option to be useful is to figure out 
> if an issue occurs because of the hostbridge emulation. Yet, I am still 
> not fully convinced adding an option is worth it.
> 
> Jan and others, any opinions?

Well, if there's a proper fallback, then why not allow using it in
case of problems?

Jan