Re: [PATCH 2/2] xen/pci: Gate all MSI code in common code with CONFIG_HAS_PCI_MSI

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.04.2021 20:06, Julien Grall wrote:
> Hi Jan,
> 
> On 06/04/2021 16:25, Jan Beulich wrote:
>> On 06.04.2021 16:30, Julien Grall wrote:
>>> Hi Roger,
>>>
>>> On 06/04/2021 15:13, Roger Pau Monné wrote:
>>>> On Tue, Apr 06, 2021 at 12:39:11PM +0100, Rahul Singh wrote:
>>>>> MSI support is not implemented for ARM architecture but it is enabled
>>>>> for x86 architecture and referenced in common passthrough/pci.c code.
>>>>>
>>>>> Therefore introducing the new flag to gate the MSI code for ARM in
>>>>> common code to avoid compilation error when HAS_PCI is enabled for ARM.
>>>>
>>>> Is such option really interesting long term?
>>>>
>>>> IIRC PCI Express mandates MSI support, at which point I don't see much
>>>> value in being able to compile out the MSI support.
>>>
>>> I am pretty sure there are board out with PCI support but no MSI
>>> support. Anyway, even if the spec may mandate it...
>>>
>>>>
>>>> So while maybe helpful for Arm PCI efforts ATM, I'm not sure it
>>>> warrants a Kconfig option, I would rather see Arm introduce dummy
>>>> helpers for the missing functionality, even if unimplemented at the
>>>> moment.
>>>
>>> ... from my understanding, most of (if not all) the MSI code is not very
>>> useful on Arm when using the GICv3 ITS.
>>>
>>> The GICv3 ITS will do the isolation for you and therefore we should not
>>> need to keep track of the state at the vPCI level.
>>
>> But that's then not "has PCI MSI" but "need to intercept PCI MSI
>> accesses", i.e. I don't think the Kconfig option is correctly
>> named. If a device with MSI support is used, you can't make that
>> MSI support go away, after all.
> 
> That's actually a good point. Rahul, do you think the config can be 
> renamed to something like CONFIG_PCI_MSI_NEED_INTERCEPT?

Minor remark: In this name I'd be inclined to suggest to omit NEED.

Jan