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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 20 Aug 2021, Jan Beulich wrote:
> 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?

I think it would be good to have the option, if nothing else for
debugging.