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

On 20/08/2021 13:19, Rahul Singh wrote:
Hi Julien,

Hi Rahul,

On 19 Aug 2021, at 1:31 pm, Julien Grall <julien@xxxxxxx> wrote:

Hi Rahul,

On 19/08/2021 13:02, Rahul Singh wrote:
Add cmdline boot option "pci=on" to enable/disable the PCI init during

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?


Julien Grall



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