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

[Julien Grall <julien@xen.org>]

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?

Cheers,

-- 
Julien Grall