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

[Jan Beulich <jbeulich@suse.com>]

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.

And of course I agree with the desire to have less #ifdef-ary
here.

Jan