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From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>, Rahul Singh <rahul.singh@arm.com>
Cc: xen-devel@lists.xenproject.org, 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: Tue, 6 Apr 2021 17:25:12 +0200	[thread overview]
Message-ID: <b0eb0833-50aa-d9a1-3265-3255970ef210@suse.com> (raw)
In-Reply-To: <75848a0d-d060-6a8b-5ebc-7376ffc14af0@xen.org>

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


  parent reply	other threads:[~2021-04-06 15:25 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-06 11:39 [PATCH 0/2] xen/pci: Make PCI passthrough code non-x86 specific Rahul Singh
2021-04-06 11:39 ` [PATCH 1/2] xen/pci: Move PCI ATS code to common directory Rahul Singh
2021-04-06 15:16   ` Jan Beulich
2021-04-06 11:39 ` [PATCH 2/2] xen/pci: Gate all MSI code in common code with CONFIG_HAS_PCI_MSI Rahul Singh
2021-04-06 14:13   ` Roger Pau Monné
2021-04-06 14:30     ` Julien Grall
2021-04-06 14:59       ` Roger Pau Monné
2021-04-06 15:09         ` Julien Grall
2021-04-06 15:58           ` Roger Pau Monné
2021-04-07 17:52             ` Rahul Singh
2021-04-06 15:25       ` Jan Beulich [this message]
2021-04-07 18:06         ` Julien Grall
2021-04-08  6:00           ` Jan Beulich
2021-04-08  8:45             ` Rahul Singh

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