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

From: Julien Grall <julien@xen.org>
To: "Roger Pau Monné" <roger.pau@citrix.com>,
	"Rahul Singh" <rahul.singh@arm.com>
Cc: xen-devel@lists.xenproject.org, bertrand.marquis@arm.com,
	Jan Beulich <jbeulich@suse.com>, Paul Durrant <paul@xen.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Ian Jackson <iwj@xenproject.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Daniel De Graaf <dgdegra@tycho.nsa.gov>
Subject: Re: [PATCH 2/2] xen/pci: Gate all MSI code in common code with CONFIG_HAS_PCI_MSI
Date: Tue, 6 Apr 2021 15:30:01 +0100	[thread overview]
Message-ID: <75848a0d-d060-6a8b-5ebc-7376ffc14af0@xen.org> (raw)
In-Reply-To: <YGxsfdK9GEefLgAv@Air-de-Roger>

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.

So I think we want to be able to compile out the code if not used. That 
said, I think providing stub would be better to avoid multiple #ifdef in 
the same function.

Cheers,

-- 
Julien Grall