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

From: Rahul Singh <Rahul.Singh@arm.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Julien Grall" <julien@xen.org>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"Bertrand Marquis" <Bertrand.Marquis@arm.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>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH 2/2] xen/pci: Gate all MSI code in common code with CONFIG_HAS_PCI_MSI
Date: Thu, 8 Apr 2021 08:45:06 +0000	[thread overview]
Message-ID: <479D17E5-3021-4A2E-B066-D179226CC48B@arm.com> (raw)
In-Reply-To: <0071311d-6da5-636b-b823-32b5570d8f49@suse.com>

Hi,

> On 8 Apr 2021, at 7:00 am, Jan Beulich <jbeulich@suse.com> wrote:
> 
> 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.
> 

OK . I will use the name CONFIG_PCI_MSI_INTERCEPT and will send next version of the patch.

Regards,
Rahul

> Jan